9. An isosceles triangle has a side of length $20 \mathrm{~cm}$. 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 $36 \mathrm{~cm}$
B $48 \mathrm{~cm}$
C $60 \mathrm{~cm}$
D $90 \mathrm{~cm}$
E $120 \mathrm{~cm}$
SOLUTION
B

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 $20 \mathrm{~cm}$ side must be one of the pair of equal sides. The base is then two-fifths of $20 \mathrm{~cm}$, namely $8 \mathrm{~cm}$. The perimeter is $20+20+8=48 \mathrm{~cm}$.