JAMB 1995 · UME · Q21

Which of the following binary operations is commutative in the set of integers?

Worked solution (try it first)

An operation is commutative when $a * b = b * a$ , that is, when swapping $a$ and $b$ leaves the formula the same.
In $a + b - ab$ , swapping gives $b + a - ba$ , which is the same, because addition and multiplication are commutative.
The others change: for example, with option A, $1 * 2 = 5$ but $2 * 1 = 4$ .
So the answer is option B.