Question 760510
y varies directly as the square of x. This means that the ratio of y to x^2 is a constant. Let us call it k. Mathematically,

{{{(y / x^2) = k }}}

y = 80 when x = 20. Substituting for y and x in the above equation,

{{{ 80 / 20^2 = k }}} or

{{{ k = 80 / 400 = 0.2 }}}

So when x = 90, 

{{{ y / 90^2 = k = 0.2 }}}

Therefore, {{{ y = 90^2 * 0.2 = 8100 * 0.2 }}} or {{{ y = 1620 }}}