This activity is on the NRICH website. Perimeter is a rich context for exploring the value of algebra as a reasoning tool. Use perimeter as a context to express sums of known and unknown quantities. The unknown values are positioned within a linear expression, and students can work backwards (solving a linear equation) to find possible missing lengths.
Use of the familiar situation of length to provide an image of the meaning of the letters provides a bridge between the missing number problems used in primary mathematics and solving equations algebraically.