5 working days in 4 weeks=5*4=20 days
money he got for regular work in 20 days=2.40*20=Rs 48.00
to find out the money he earned by overtime, Rs 432-Rs 48 =Rs 384
to find the hours he worked overtime, total money earned / money in 1 hour=384/3.20=120
therefore,in case of regular work, working hours are 8, so, 20*8=160
the total hours he worked=120+160=280
i tried my best,i don't know whether its correct!!!