Question

A completes a works in 15 days and B completes the same work in 20 days.
If they both work together in 4 days .
how much work is left?

Answer 1

rate if A = 1/15
rate of B = 1/20
combined rate = 1/15 + 1/20 = 7/60

time at combined rate = 1/(7/60) = 60/7 days
but they only worked for 4 days,
so the time left is 60/7 - 4 or 32/7 days
and (32/7) / (60/7) = 32/60 or 8/15 of the work
So 8/15 of the job still have to be done

or

in terms of time, they still have to complete
appr 4.57 days of work