Assume 60 units of work..

A can do in 20 days means he does 3 units per day..
B can do in 30 days means he does 2 units per day..

they work togetehr, they do 5 units per day..

let us say it took D days to compete project
So 5(d-10)+2(10) = 60
therefore, d = 18

what is OA?

a in 1 day 1/20
b in 1 day 1/30
a+b in 1 day 1/20+1/30=1/12
so a+b together will complete work in 12 days
now a leave10 days before project is completed
so a+b work together for 2 days
in 2 days a+b will complete 1/12*2=1/6 work
so work remaining 1-1/6=5/6 of work
now b in 1 days does 1/30
so b will do 5/6 of work in another (5/6)/(1/30)=25 days
so project will be 25+2=27 days

amising6 wrote:a in 1 day 1/20
b in 1 day 1/30
a+b in 1 day 1/20+1/30=1/12
so a+b together will complete work in 12 days

Hi Aimsing6,

A left the work 10 days before the project completion date...
But not 10 days before expected completion date..

be careful about the detail!!

Both A and B can complete 1/12 unit of work in 1 day

For(d-10) days,they complete 1/12*(d-10) unit of work

Next 10 days B alone complete 1/3 work.

1/3+1/12(d-10)=1

d=18