$990
550 x .80 = 440
$550 + $440 = $990

We effectively rewrite the equation as

In order for the LHS to be defined, we need to restrict
, or
. Now, the LHS will vanish when the numerator is 0, which happens for

This value is indeed smaller than 4, so the solution is
.
r= 9 because 45/5 is 9 and it works the same way for the rest of the problems.
Positive because it does not go less than 0.