Find a side from the perimeter, step by step
A rectangle's perimeter is the two sides doubled, so halving it gives back the length plus the width.
The question
cm
Missing side: perimeter 50, one side 18
2 × (18 + ?) =50
50 ÷ 2 =25
50 ÷ 2 =25
25 − 18 =7 cm
- Forwards, the perimeter goes round twice: 50 is 18 + ? + 18 + ? — each side counted two times.
- So undo the doubling FIRST: half of 50 is 25, and that is one long side plus one short one. Taking 18 off the whole 50 is the natural wrong move — it forgets that 18 is in there twice.
- Now take the side you know off that half: 25 − 18 = 7 cm.
How it works.
- 01
Forwards, the perimeter goes round twice: 50 is 18 + ? + 18 + ? — each side counted two times.
- 02
So undo the doubling FIRST: half of 50 is 25, and that is one long side plus one short one. Taking 18 off the whole 50 is the natural wrong move — it forgets that 18 is in there twice.
- 03
Now take the side you know off that half: 25 − 18 = 7 cm.