Question

A is thrice as fast as b and together can do work in 21days

Answer 1

A= 3x
days=21
B=21÷3
=7 days

Answer 2

If B can do x amount of work in 21 days then A can 3x amount of work in 21 days.
Solve for x: A + B = 21 days therefore 3x + x = 21 days
4x = 21 days
x = 21/4 days
x = 5 1/4 days

If B is not there then A has to do their work and B's work therefore it will take A 21 days + 5 1/4 days = 26 1/4 days