It’s 7km from Tim’s house to where his Grandma lives. It’s uphill on the way there so he can only cycle at 14kph but he can do 21kph on the way back.
Tim leaves his house at 6pm. How long can he stop at his Grandma’s house if he wants to be back home for his favourite TV programme at 7pm?
Solution
The trip to Grandma’s takes 30 minutes. (If he were travelling at 7kph he would take an hour. Travelling at twice that speed halves the time.) This means that the home trip takes a third of an hour or 20 minutes. Tim’s total travelling time is therefore 50 minutes. If he spends 10 minutes with Grandma he can still get home to watch his favourite programme.
Extension
At what time should Tim leave home if he wants to spend 45 minutes at his Grandma's?