Well it is simple.If he was able to solve 400 problems in just 25 ' then how long would it take him to solve 100(1/4 of 400)?It would take him 6.25' to solve 100 problems(1/4 of 25).So if he had to do another 500 (because 1000 -500=500) it would take him 31.25' (5*6.25) to complete them.If you have any further questions please contact me.
