A can do a piece of work in 25 days which B alone can do it in 20 days. A started the work and was joined by B after 10 days. Find the time taken to finish the

Question

A can do a piece of work in 25 days which B alone can do it in 20 days. A started the work and was joined by B after 10 days. Find the time taken to finish the work.

Answer 1

Pretend for a moment that there are 100 units of work. (I picked this number because 25 and 20 go evenly into 100.)

 

If A can do the entire job in 25 days, then A does 100/25 = 4 units per day.

If B can do the entire job in 20 days, then B does 100/20 = 5 units per day.

Together they can d 4+5 = 9 units of work per day.

 

If A worked for 10 days alone, then A did 10*4 = 40 of the 100 units, leaving how many?

 

How many days does it take to do the remaining units, at 9 units per day?