A bag contains 8 red balls, 5 green balls and 7 yellow balls.

1. Find the probability of choosing a ball that is not red.

Total number of balls:

$8+5+7=20$

Number of balls that are not red:

$5+7=12$

So,

$P(\text{not red})=\dfrac{12}{20}=\dfrac{3}{5}$

1. Form and solve the probability equation after removing $x$ yellow balls.

Total number of balls originally:
$8+5+7=20$
After removing $x$ yellow balls, total number of balls is
$20-x.$
The number of green balls remains $5$, so
$\frac{5}{20-x}=\frac{1}{3}.$
Solving,
$15=20-x$
$x=5.$