When 'y' is directly proportional to 'x', we express this by
y = kx
where 'k' is the constant, which we need to work out first
Cost of circular mirrors = (k) (diameter)²
50 = k(60)²
50 ÷ 3600 = k
the proportional formula is
y = (1/72)x²
The cost of the mirror when diameter = 90cm
y = (1/72) (90)²
y = 112.5
Answer: $112.50