Always Two

Why use this problem?

The problem gives practice in writing equations from verbal information and in algebraic manipulation. Learners will experience the value of recognising and making use of the symmetries in the algebraic expressions that occur.

As a non-standard problem, it is designed for learners to think for themselves, but it does not require any mathematical knowledge beyond knowing how to solve two linear simultaneous equations in two unknowns.

Possible approach

Encourage learners to work in pairs or small groups to discuss how they might tackle the problem, then to work out the solutions individually, and finally to check together if their answers agree.This is reassuring for people who are inclined to panic at the unfamiliar, and gives practice in communication of mathematical ideas.

Learners may find one or both of the solutions by trial and error, but the challenge is to then prove that there are no other solutions.

Key questions

Are you using the symmetry of the expressions?

If you subtract one equation from another in pairs, what do you notice?

What can you deduce if the product of two linear factors in an equation is zero?

Possible support

In order to gain some experience of thinking for themselves, and not simply following set procedures to solve a system of equations, the class could initially try the problem Symmetricality which is another non-standard problem involving simultaneous equations. 

As practice in solving more standard sets of simultaneous equations with unit coefficients in more than 2 unknowns learners could try Simultaneous Equations Sudoko

Possible Extension

Try Leonardo's Problem where you have to first create the equations, and then solve them.