Let M mean Michael, and B mean Brandon, when write two equations:
M = 4B
B = M - 27
Replace the B in the first equation with what it is equal to in the second equation:
M = 4(M - 27)
Distribute
M = 4M - 108
Subtract 4M from both sides
-3M = -108
M = 36
Now put that back into the equation for B.
B = M - 27
B = 36 - 27
B = 9
So, Brandon is 9 and Michael is 36.
