Units of time

Time is counted in sixties

With length and weight the steps were 10, 100, 1 000. Time is a rebel — its step is 60:

• 1 minute (min) = 60 seconds (s),
• 1 hour (h) = 60 minutes,
• 1 day = 24 hours.

Why sixty? It is an ancient agreement — sixty splits beautifully (into 2, 3, 4, 5, 6, 10, 12…), so halves, quarters and thirds of an hour come out as whole minutes.

And exactly those pieces are used all the time: a quarter of an hour = 15 minutes, half an hour = 30 minutes, three quarters = 45 minutes.

Watch the biggest trap of calculating with time: the tens overflow does not work here. 1 h 70 min is not a correct write-up — 70 minutes is more than an hour, so correctly it is 2 h 10 min. When calculating with time you “pour over” in sixties, not in hundreds.

Today you will learn to convert time, add it and subtract it — say work out when a film ends.

Jonáš wants to catch a film in the evening. It starts at 19:30 and lasts 1 h 45 min. “What time does it finish?” mum asks, “at nine you have to be in bed.” Jonáš counts: 19:30 plus an hour is 20:30. Plus 45 minutes… 30 minutes takes him to 21:00 and 15 is left to add: 21:15. Mum shakes her head: that is after nine. Jonáš quickly looks for an earlier start: if the film started at 19:00, it would finish at 20:45. The bargaining starts — but at least he has exact numbers in his hand.

Conversions and calculating with sixty

Conversions:

• h → min: times 60. Example: 2 h = 120 min.
• min → s: times 60. Example: 3 min = 180 s.
• min → h: divided by 60, the remainder is minutes. Example: 90 min = 1 h 30 min (90 ÷ 60 = 1, remainder 30 — division with a remainder in action!).

Adding times: add hours on their own and minutes on their own. When minutes climb over 60, take away 60 and add an hour.

1 h 40 min + 50 min = 1 h 90 min → 90 min is 1 h 30 min → 2 h 30 min.

How much has passed (from–to): count on through whole hours, like a shopkeeper with change.

From 8:45 to 10:20:

1. From 8:45 to 9:00 → 15 min.
2. From 9:00 to 10:00 → 1 h.
3. From 10:00 to 10:20 → 20 min.
4. Together: 1 h 35 min.

Quarters of an hour in words: “quarter past seven” is 7:15, “half past seven” 7:30, “quarter to eight” 7:45. The words talk about the hour you are heading towards.

Converting minutes to hours and back

Convert: 3 h to minutes and 100 min to hours and minutes.

3 h → min: 3 × 60 = 180 min.

100 min → h and min: I divide with a remainder: 100 ÷ 60 = 1, remainder 40. So 1 h 40 min.

Check: 1 × 60 + 40 = 100. It fits.

See what division with a remainder was for? Time uses it all the time — whole hours are the quotient, minutes the remainder.

When the film ends

A film starts at 19:30 and lasts 1 h 45 min. When does it end?

Step 1: I add an hour: 19:30 + 1 h = 20:30.

Step 2: I add minutes in pieces: 20:30 + 30 min = 21:00 (a whole hour, the anchor). 45 − 30 = 15 min is left.

Step 3: 21:00 + 15 min = 21:15.

A trap you avoided: 30 + 45 = 75 minutes must not be written as “20:75”. Seventy-five minutes is an hour and a quarter.

How long the journey lasts

A train leaves at 9:50 and arrives at 11:35. How long does it go?

I count on through whole hours:

1. From 9:50 to 10:00 → 10 min.
2. From 10:00 to 11:00 → 1 h.
3. From 11:00 to 11:35 → 35 min.

Together: 1 h + 10 min + 35 min = 1 h 45 min.

Check the other way: 9:50 + 1 h 45 min → 10:50 + 45 min → 11:35. It fits.

Practice: now you do it 🎲

Now try it for real. You will see all the questions at once, like on paper. Start with ten. When it goes well, add more.

Convert hours, minutes and seconds. The step is 60 — division with a remainder helps.

Practise the questions

On paper

Calculate with a real timetable — time is most useful where you live.

1. Convert: 2 h to minutes, 5 min to seconds, 130 min to hours and minutes, 200 s to minutes and seconds.

2. A school lesson lasts 45 minutes. How many minutes are two lessons with a ten-minute break between them? Write it in h and min too.

3. Training starts at 16:15 and lasts 1 h 30 min. What time does it finish? Count in pieces through a whole hour.

4. Measure (with a stopwatch on a parent’s phone) how many seconds you can stand on one leg. Convert to minutes and seconds if you go over sixty.

Now you 💪

1. You know that an hour has 60 minutes and a minute 60 seconds — and that it does not pour over in hundreds.
2. You convert 100 min to 1 h 40 min by dividing with a remainder.
3. You work out the end of a film: 19:30 + 1 h 45 min = 21:15.

What to take from this lesson

• Time has step 60: a minute = 60 s, an hour = 60 min. A day has 24 h.
• Minutes over 60 pour over to hours: 90 min = 1 h 30 min.
• Add in pieces up to a whole hour — it is an anchor.
• Duration from–to: count on through whole hours like a shopkeeper.
• Quarter past seven = 7:15, half past seven = 7:30, quarter to eight = 7:45.

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