Here is a question from a booklet that I cannot do can anyone help?

题目

Here is a question from a booklet that I cannot do can anyone help?
Sandy and David decide to race each other for 100 metres.David wins by 10 metres.They decide to race again,but this time,to even things out,David begins 10 metres behind the starting line.Assuming that both boys run at the same constant speed as before,who will win the second race?

答案

David againBecause when David runs 100 metres Sandy can only runs 90 metres.At the second time ,they come to the point that 10 metres left at the same time .At the last 10 metres ,David runs faster.