A work can be completed by 20 men in 50 days. 30 more men join them after 10 days of work. How many days will be saved in completion of the work?

A work can be completed by 20 men in 50 days. 30 more men join them after 10 days of work. 

When 30 more men join them in the work, then the time taken by them to complete the remaining work is 'y' days.

$$men_1\times\ days_1\ =\ men_2\times\ days_2$$

$$20\times50\ =\ 20\times10+\left(20+30\right)\times y$$

$$1000\ =\ 200+50\times y$$

$$1000-200=\ 50\times y$$

$$800=\ 50\times y$$

y = 16

So in the second case, the total time taken = 10+y = 10+16 = 26 days

Hence the number of days will be saved in completion of the work = 50-26 = 24 days