assuming that all the bees do an equal amount of work in an equal time span.
(a) 40 bees finish 1 work in 30 hours.
=> (b) 40 bees will finish 1/30 work in 1 hour.
=> (c) 1 bee will finish 1/1200 work in 1 hour.
=> (d) 1 bee will finish 1 work alone in 1200 hours.
considering statement (b):
40 bees will finish (10/30=)1/3 work in 10 hours.
remaining work to be done= 1-1/3 = 2/3 work, and
remaining number of bees= 40-15 = 25 bees
Now, 25 bees have to do the remaining (2/3)rd work.
considering statement (d):
25 bees will finish 1 work in = 1200/25 hours.
So, 25 bees will finish remaining (2/3)rd work in = (1200/25) * (2/3) = 32 hours