for 1 hour he will do 1/y work...
for 2 hour he will do 2/y work...
So for 3 hours he will do 3/y work...
remaining work= 1-(3/y)
therefore answer is (y-3)/y.
Maths is just like a stone if u will uttern it therefore u can acheive the zenith of success but if u will not in that case it will become headache for u...