Hours, minutes, seconds

In your book, Maths Mela (Chapter 12), Zainab and Remo change hours into minutes and back. They know that 1 hour = 60 minutes. So 2 hours is 2 × 60 = 120 minutes, and 3 hours is 3 × 60 = 180 minutes, which means 180 minutes = 3 hours.

A double number line helps. On the top line write the hours 1, 2, 3, 4 …; on the line below write the minutes 60, 120, 180, 240 …, going up by 60 each time. To change 2 hours 45 minutes into minutes, find 2 hours on the line (120 minutes) and add the 45: 120 + 45 = 165 minutes. To change 390 minutes into hours, find the biggest number on the minutes line that is not more than 390: that is 360, which is 6 hours. 390 − 360 = 30 minutes are left, so 390 minutes = 6 hours 30 minutes.

Seconds work the same way, because 1 minute = 60 seconds. 320 seconds: 5 minutes is 5 × 60 = 300 seconds, and 20 seconds are left, so 320 seconds = 5 minutes 20 seconds. The other way: 10 minutes 13 seconds is 10 × 60 = 600 seconds, plus 13, which is 613 seconds.

Raghav studies 4 subjects: Mathematics, English, Hindi and The World Around Us, for 50 minutes each. Altogether that is 4 × 50 = 200 minutes. 180 minutes is 3 hours, and 20 minutes are left, so he studies for 3 hours 20 minutes.