Question

A can do a work in 15 days and b in 20 days. if they work on it together for 4 days, then the fraction of the work that is left is:

a.1/4

b.1/10

c.7/15

d.8/15 answer: d

Answer 1

suppose total work is 1

A=1/15 & B=1/20 for one day work

when both work together

(A+B)= (1/15)+(1/20)=7/60

for 4 days

(7/60)x4=7/15

for remaining work

1-(7/15)=8/15

Answer (8/15)

Answer 2

A's 1day work=1/15
B's 1day work =1/20
Both work can in 1day =1/15+1/20=35/300=7/60
They work for 4 days =4*7/60=28/60=14/30=7/15
Now left =1-7/15=8/15