PK 2020 Q9
9. An isosceles triangle has a side of length 20 cm. Of the remaining two side-lengths, one is equal to two-fifths of the other. What is the length of the perimeter of this triangle?
- A 36 cm
- B 48 cm
- C 60 cm
- D 90 cm
- E 120 cm
Pink Kangaroo 2020, Question 9. © UK Mathematics Trust. Original paper on ukmt.org.uk
Answer: B 48 cm
The triangle is isosceles so has a pair of equal sides. Since the two unknown sides are not equal, they cannot be the pair of equal sides, and hence the 20 cm side must be one of the pair of equal sides. The base is then two-fifths of 20 cm, namely 8 cm. The perimeter is 20 + 20 + 8 = 48 cm.
Solution © UK Mathematics Trust, from the PK 2020 solutions.