26 1/4 days

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