Answer:

Given:

A can complete the work in 10 days

B can complete the work in 15 days

Concept used:

Total work = LCM of  time taken

Efficiency = Total work/Total time

Calculation:

LCM of (10, 15) = 30

Let the total work be 30 units

According to the question:

The amount of work completed by B in 5 days,

⇒ 2 × 5 = 10 units

Remaining work after 5 days,

⇒ 30 – 10 = 20 units

Now A and B work together

Then the total efficiency of A and B,

⇒ 3 + 2 = 5 units/days

Time taken to complete the work when A and B working together,

⇒ 20/5 = 4 days

The time taken to complete the work

⇒ 5 + 4 = 9 days

∴ The total time take to complete the work is 9 days 

Answer:

LCM of (10, 12, 15) = 60 units

→ The efficiency of A = 6 units

The efficiency of B = 5 units

→ The efficiency of C = 4 units

First 2 days (A + B + C) = (6+5+4)* 2 => 30units

Last 4 days work done by B alone =

5 x 420 units

Remaining Work = 60 - (30+20) - 10 units

The remaining work is completed by B and C = 10/(5+4) → 10/9 days

So total number of days = 2 + 4 + 10/964/9 days

.. Work was finished in 64/9 days

hope this helps ❤️