How many minutes are there in 2½ hours? With metres and centimetres we just slide the decimal point along. Can we do the same trick here, or does time behave differently?

Look at the clock. The second hand goes all the way round, a full sixty seconds, before the minute hand moves on by just one notch.

The minute hand goes all the way round, a full sixty minutes, to make one hour.

A whole day is twenty-four of these hours.

It takes lots of small units to fill one big unit, so these jumps are 60s and 24s, not tens. That is why we cannot slide a decimal point here. Because it takes many small units to make one big one, to change a big unit into a small one we multiply. To change a small unit into a big one we divide.

First let's build 2½ hours on the board by hand. Two whole hours is 2 × 60 = 120 minutes. The half hour is ½ × 60 = 30 minutes. Add them: 120 + 30 = 150 minutes. So 2½ hours is 150 minutes, not 25 — we multiplied, we didn't slide the point.

Now onto the × 60 machine. Before we feed 3 hours through, predict: how many minutes will come out? Because it takes many small minutes to make one big hour, going hours to minutes means we multiply.

Then swap the rule to ÷ 60 and feed those minutes back through. Where does it land? Going the other way, minutes to hours, we divide.

Now let's crack these on the board in order. First take 180 minutes back to hours. Next turn 3 days into hours. Then convert 2 hours into minutes. Last comes the film stretch. Those 120 minutes from a 2-hour film, how many seconds is that? Predict multiply-or-divide before each one goes through.