50% off what

Fifty percent of what?

50% off sounds like a win. Without the question of what, it is magic. 50% of 400 that nobody wanted is 200 for a thing that is 90 in another shop. The eyes celebrate. The balance does not.

A percent is school. Here this is enough: half of the base. The base must exist. “It was 399, now 399” with a sign −50% is a lie, not maths.

Jakub sees a hoodie −50%, 600→300. Tereza: “Would you wear it with no sign?” No. 300 from want kills the goal. Jakub: “But it is half!” Half of a mood he did not want. Tereza walks away. She is not a miser. She is Shopper.

A web shop: 50% off after registering an email. A child’s email does not belong here. A discount that wants a mailbox is not a discount. It is collecting.

The base, the new number, a want

Three steps. 1) What was the price it is taken off? 2) What is it now? 3) Would I want it at the new number even if the −50% sign was not there? Step 3 is value from التحضير. Without it a percent is hypnosis.

50% of 80 is 40. Halving can do that. 30% is harder — on pocket money “about a third” is enough. Leave precision to adults on big things. You train eyes.

A discount with time (“until midnight”) is pressure. It comes separately. Today: even a true half may wait until morning, when the envelope is asleep.

Balance: 100 with a discount, when you have 60 in want, still does not go. A discount must not open a loan from a friend.

Tereza wants to shorten it to “it will be fine somehow”. Jakub asks what the lesson 50% off what is for, when a table can do a sum. A sum does not count respect, a PIN, a trap or an envelope. The core is: 50% counts only when you know the base and want the new number with no sign too.

Adults pay the rent. The card, the account and the PIN belong to a parent. A child’s email does not belong here. Pocket-money numbers do not belong on the network. If you break these rules in the lesson “50% off what”, you know a word, not the course.

A toy 400 → 200 (−50%). Next to it the same with no brand 120. Jakub takes 200 “because a discount”. Tereza 120 or nothing. 50% off lost to 120.

For practice: 50% of 90 = 45. If the envelope cannot carry 45, the percent does not matter.

Practice numbers, not a bank statement. Tereza has 80 Kč in the want envelope. Jakub pushes a step this lesson can name. When Tereza uses the core, either it waits for a parent, or it fits the envelope. If not, anger and a hole are left — proof of pressure, not that the lesson is pointless.

Why this right now

You are in the course Money with Martin, level Shopper. You can write numbers in a notebook or a table, but the topic is money and decisions, not Excel formulas. The account, the card and the PIN belong to a parent. A child’s email does not belong here. Do not put pocket-money numbers on the network.

This is not the Sheets with Martin course. A table may help, but the mark of success is a decision, not SUM.

You will know this lesson by one sentence: 50% counts only when you know the base and want the new number with no sign too. When you can say it to a parent or a teacher, you have the core. The rest is practice, not magic.

Free postage and a subscription can hide a price. First see the percent. Then hidden spending.

If you skip this, the next lesson in level Shopper will stand on fog. You can say the base and the new number. Step 3 can say no. It is not a test for marks. It is a safety net, so “50% off what” is not only a heading.

Four things you will build from the course

The course has four small projects. You do not have to do them all at once. Today you add only what this lesson can do.

Workshop: a base on paper

One discount from a shop window or a practice one. Count a half. Then step 3.

1. Write the base and the new number. Is the base missing? The discount does not count as information.

2. A half of practice 80, 200, 90. The head, not a formula as a mark.

3. A question: would I want it at the new number with no sign? Yes/no.

4. Do not send an email for a code. A parent, if at all.

You count when you can say the base. A sign with no base is a poster. A poster does not go into an envelope.

Pocket money: Pocket money: a discount that empties later is not a win. Do not write someone else’s amount, a PIN, an IBAN or “how much a classmate gets” next to it. A rule and a practice number are enough — or an honest zero.

Class trip: A trip: −50% a souvenir you do not want still takes the till. Do not write someone else’s amount, a PIN, an IBAN or “how much a classmate gets” next to it. A rule and a practice number are enough — or an honest zero.

Practice: a half, or hypnosis?

Count. Then decide.

1. 50% of 200. The new number? Step 3, when you do not want it?

2. A sign −50% with no crossed-out old price. What is missing?

3. A discount for a child’s email. Why not?

4. 100 with a discount, want envelope 60. What wins?

Now you 💪

Go through three checks. By each one, tick in your head: do I see it, or not?

1. I ask of what, not only how many percent.
2. I can do a half in my head with round numbers.
3. A sign will not order a purchase against the envelope.

If something is missing, go back to the steps above. It is not a test for marks. It is a safety net, so you go on with certainty, not with a mess.

What to take from this lesson

• A percent with no base is fog.
• Do you want it even with no sign? If not, the discount does not lie about you — about a mood.
• No email and PIN for a discount.
• The envelope > a percent.
• الاسمs in the examples (Tereza, Jakub) are made up. Do not put your number, email or photo here.
• Next lesson: When you can do this, go to the next lesson in the same level.

© 2026 Ing. Martin Polak / AlgoRhino · شروط استخدام المحتوى