7,35,210,___,6300,31500 ???

7*5=35 35*6=210 210*5=1050 1050*6=6300 then answer will be 1050.

lf a man walks 3/5th of his original speed and gets 12 minutes late. What is his usual speed? In this question since distance is constant, speed and time will be inversely proportional. So if speed decrease by 2/5 then the time increases by 2/5. Hence 2/5*T=12 T=12*5/2=30 minutes what is wrong with this approach?

yes. the first line is the question. what follows is my doubt.