Question

A and B can do work in
15 days and 20 days respectively
If A starts working and B join
after three days, find the number
of days taken by both to
complete the work.​

Answer 1

Solution

Time taken by A to complete the work = 15 days

∴ work done = 1/15

After15 days B join A

1/15 × 3 = 1/15

Remaining work = 1 - 1/5

Time taken by B to complete the work = 20 days

Work done by B = 1/20

If they works together =

1/15 + 1/20

=> 4+3/60

=7/60.

Remaing work done by B =

7/60 ÷ 4/5

= 7/48

∴ Time taken by both to complete the work =

1 ÷ 7/48

$\large \underline{\boxed{ \bold{\implies6 \frac{6}{7} \: \: \: \: \: answer}} }\:$

Answer 2

Answer:

A and B do a work in 20.

In other words, the fraction of work done by

A and B in 1 day is 1/20.

similarly fraction of work done by

B in 1day is 1/15

Step-by-step explanation:

NOW.

A and B work together for 2 days and

and

complete 2/20 or 1/10 of work

work left is 1-1/10 = 9/10

which B has to do

1/15 of work is done by B in

1 day so 9/10 of its complete by B in

9×15/10

=135/10=13.5 day