This is because, if he uses 40% of his exercise on the treadmill, which he spends 40 minutes on, it means that 10% of his exercise takes 10 minutes (40% ÷ 4 is 10%, and 40 mins ÷ 4 is 10 minutes). To find 100% (his whole exercise routine) we have to do 10 mins x 10, which 100%. This means that he exercises for 100 minutes each day.