Let a and b be the respective rates in bricks per minute for Alex and Bob separately.
Alex lays 4 bricks per minute more than Bob
a = b + 4
Working together, their rates scale by 3/4 and they achieve 15 bricks per minutes.
(3/4)(a + b) = 15
Two equations, two unknowns, we solve:
b = a - 4
(3/4)(a + a - 4) = 15
2a - 4 = (4/3) 15 = 20
2a = 24
a = 12
b = a - 4 = 8
Answer: Alex alone does 12 bricks per minute, Bob alone 8 bricks per minute
Check: a=b+4, good
(3/4)(12 +8)=(3/4)(20)=15, good