Can Do Work In 15 Days And B In 20 Days

A can do a work in 15 days and B in 20 days. If they together work on it for 4 days what fraction of the work will be left ?

Answers:

A’s 1 day work = 1/15

B’s 1 day work = 1/20

(A + B)’s 1 day work = 1/15 + 1/20

   = (4 + 3)/60 = 7/60

(A +B)’s 4 days work = (7/60) × 4 = 7/15

Remaining work = 1 – (7/15) = (15 – 7)/15

= 8/15