Answer:
12 minutes
Explanation:
You have to think about this as if it were a fraction where it is
4 minutes x minutes
------------------ and -----------------
7 kilometres 21 kilometres
As you can see on the bottom row you can divide the 21 kilometres and the 7 kilometres to see what it took to get there and if you were to do that it would be 3. So now that we have the 3 we are going to multiply that by the 4 minutes so that we can see how many minutes it would take for 21 kilometres and the answer is...
3 x 4 = 12