Question

A can do a piece of work in 30 days while B can do it in 40 days. In how many days can A and B working together to do it?

Answer 1

let the total work be 120 units( lcm of 30 and 40)
so one day work of A is 4 unit
and that of B is 3 unit.
so total work done by A and B in one day is 3+4 = 7 units.
time to complete the work is
$\frac{120}{7} = 17 \frac{1}{7}$

Answer 2

A do a piece of work in 30 days.
B do the piece of work in 40 days.
when A and B working together then time will be - working days of A× working days of B
______________________________
working days of A+working days of B
total days =30×40/30+40
=1200/70
=120/7
=17 1/7 days.....