In demonstrating how a 2-pan scale works, we notice that a brick placed on one side exactly balances 3/4 of a kg plus 3/4 of a brick on the other side. If this is so, what does the brick weigh?
Let b represent the weight of the brick.
Then b = 3/4 + 3/4b → 4b = 3 + 3b → b = 3.
Check: 3 = 3/4 + 3/4 · 3 = 3/4 + 9/4 = 12/4 = 3.