A machine starts working at 21 50 and runs without stopping for 640 minutes. Write down the time, on the 24-hour clock, at which the machine stops.

The duration is given in minutes, so convert it to hours and minutes before adding it to a clock time. Dividing by 60 does this: \(640 \div 60 = 10\) remainder \(40\).

640 minutes \(=\) 10 hours 40 minutes [M1]

Now add to the start time in two steps. Adding 10 hours to 21 50 gives 31 50 on a 24-hour count, which is past midnight, so subtract 24 hours to get 07 50 the next day. Adding the remaining 40 minutes to 07 50 gives 08 30, because \(50 + 40 = 90\) minutes \(=\) 1 hour 30 minutes.

The machine stops at 08 30. [A1] cao

Two things are easy to get wrong. First, \(640 \div 60\) on a calculator shows \(10.6\overline{6}\), and the decimal part \(0.6\overline{6}\) is \(40\) minutes, not \(66\) minutes; multiplying \(0.6\overline{6} \times 60\) recovers the \(40\). Second, the answer must be written in 24-hour form with four digits, and here it falls on the following day.