I desperately need help on this impossible conundrum of an equation.

Question

Noah ordered a meal delivery from a local restaurant. Noah gave the delivery person $20 to pay for his meal, including a tip. What percentage of the price before tax did he tip the delivery person?

Answer 1

if the tip = $2

then 2/18 = 1/9 = about 11% tip

if the tip were $3

then 3/17 = about 18% tip

if the tip were $4

then 4/16 = 1/4 = 25% tip

if $5 tip

then 5/15 = 1/3 = about 33% tip