Question

If $I$ varies inversely as $d^2$ and $I=20$, when $d=3$, what is $I$ when $d=10$?

• A. $6$
• B. $66\frac{2}{3}$
• C. $18$
• D. $1.8$
• E. $12$

Answer 1

The correct option is D $1.8$

Given that $I$ varies inversely with $d^2$

Let the proportional constant be $k$.
We get $I=\frac{k}{d^2}$
At $I=20$, $d$ is $3$, so we get $k=20\times 9=180$
So, the equation becomes $I=\frac{180}{d^2}$
When $d=10$, we get $I=\frac{180}{100}=1.8$