Praful has done 1/2 of a job in 30 days, Sarabjit completes the rest of the job in 10 days. In how many days can they together do the job?

Let total work to be done = 60 units

Praful has done 1/2 of a job i.e. $$\frac{60}{2} = 30$$ units in 30 days

=> Praful's efficiency = $$\frac{30}{30} = 1$$ unit/day

Work left = 60 - 30 = 30 units which is done by Sarabjit in 10 days

=> Sarabjit's efficiency = $$\frac{30}{10} = 3$$ units/day

Praful and Sarabjit 1 day's work = 1 + 3 = 4 units/day

$$\therefore$$ Time taken by them together to complete the work = $$\frac{60}{4} = 15$$ days