Joe earned $540 at his job and paid $81 in taxes. how much tax will he pay if he earns $620?

$93

Steps:

Divide $81 in taxes by the $540 he made to find the percentage (in decimal form) of taxes he pays;

81/540 = 0.15

And then use this decimal and multiply it by $620 to find out how much he would pay in taxes at this earning;

620 * 0.15 = 93