The brightness of a lamp varies inversely as the square of the distance from it. At 2 m the brightness is 45 units. What is it at 3 m?

Explanation

1. 01

   $B \times d^2$ is constant: $45 \times 2^2 = 180$.

2. 02

   At 3 m: $B = 180 \div 3^2 = 180 \div 9 = 20$ units.