How do you do the math for a brick that balances with three quarters of a brick and three quarters of a pound?

1 brink = (¾+¼) brick = ¾ brick + ¼ brick

If 1 brick balances ¾ brick + ¾ lb then:

1 brick = ¾ brick + ¼ brick = ¾ brick + ¾ lb

→ ¼ brick = ¾ lb (subtracting ¾ brick from both sides)

→ 1 brick = 3 lb (multiplying both sides by 4).