A man driving at 4/5th of his original speed reaches his destination 15 mins later than the usual time . The usual time ( in minutes ) is  :

60

Ratio                            old     :      new

Speed                          5        :        4

Time                            4         :       5

According to question , 1R = 15 min

                                 4R  = 15 × 4 = 60 min

So , usual time is = 60 min