TimeGrades 1–6
A walk starts at 4:50 and lasts 25 minutes: when does it end? In these tasks a child adds and takes away time, and the catch is that an hour has 60 minutes, not 100. Going past the whole hour, past noon or past midnight is where most mistakes happen.
4:50 + 10 min = 5:00 5:00 + 15 min = 5:15
Not 100. So 8:50 and 20 minutes more is 9:10, not 8:70, and 110 minutes is 1 hour 50 minutes, not 1 hour 10.
To add time across an hour, step to the whole hour first and add the rest after it: from 8:50, 10 minutes to 9:00, then 10 more.
After 23:59 comes 0:00 of the next day. From 22:00 to 6:00 is 2 hours to midnight and 6 hours after it.
How long from one time to another? Stop at the whole hour on the way. It turns one hard subtraction into two easy steps.
| From and to | Through the whole hour | How long |
|---|---|---|
| 8:40 to 9:15 | 20 min + 15 min | 35 min |
| 10:50 to 12:30 | 10 min + 1 h 30 min | 1 h 40 min |
| 22:30 to 6:00 | 1 h 30 min + 6 h | 7 h 30 min |
Draw a line and put the times on it in order, the earlier on the left. Step to the whole hour, then on by whole hours, then add the minutes that are left. The examples below go from the simplest to an olympiad's.
Example 1 · Grades 1–2
Tom's swimming lesson starts at 3:45 and lasts 40 minutes. At what time does it end?
Answer At 4:25.
Example 2 · Grades 3–4
A chess tournament has 5 rounds. Each round lasts 50 minutes, and there is a 15-minute break between every two rounds. The first round starts at 10:00. At what time does the last round end?
■ ■ ■ ■ ■ rounds □ □ □ □ breaks
Answer At 15:10.
Example 3 · Grades 5–6
Grandma flies to see Ann, who lives in another city. The plane takes off at 8:00 by the clocks of Grandma's city and lands at 8:30 by the clocks of Ann's. The clocks in Ann's city show 4 hours less than in Grandma's. How long is the flight?
Grandma 8:00 → 12:30 Ann 4:00 → 8:30
Answer 4 hours 30 minutes.
MathTrail explains every wrong answer by the trap it fell into. These are the commonest in this topic, with something you can say to your child.
The child does an action the question does not ask for: adds two clock times as if they were amounts, 3 o'clock and 5 o'clock making 8, or multiplies where the times only need counting on. A time on the clock is a point on a line, not an amount.
What to say
“Is this a time on the clock or a length of time?”
Minutes are counted as if an hour had 100 of them: 3:45 and 40 minutes more becomes 3:85, or 310 minutes becomes 3 hours 10 minutes.
What to say
“How many minutes are there in an hour?”
Earlier and later change places: the child counts back when the question goes forward, or adds a time difference that should be taken away, and the plane lands before it takes off.
What to say
“Should the answer be earlier or later than the start?”
Keep a clock with hands where the child can see it, and ask in passing: what time will it be in a quarter of an hour? And what was it twenty minutes ago?
Before a cartoon or a walk, guess together how long is left to wait, then check on the clock. A real wait makes the minutes count.
Write a plan of a free day with times: a walk from 11:30 to 12:15, lunch at 12:30. How long is each part, and how long is the gap between them?
