Find a side from the perimeter
The question
cm
Missing side: perimeter 50, one side 18
How it works.
2 × (18 + ?) =50
50 ÷ 2 =25
50 ÷ 2 =25
25 − 18 =7 cm
- 01
The perimeter goes around every side: 50 is 18 + ? + 18 + ? — each side counted two times.
- 02
So halve it first: half of 50 is 25, which is one long side plus one short one.
- 03
Now take the side you know off that half: 25 − 18 = 7 cm.