Explanation:
first of all, if he continuously earns only $160 per month (or at least in that dimension), then he will not have to pay any income tax.
but to do the exercise :
100% = $160
1% = 100%/100 = 160/100 = $1.60
10% = 1% × 10 = 1.6 × 10 = $16
he earns after tax
160 - 16 = $144
FYI :
when we combine all the calculations above into one sequence, then we know
100% - 10% = 90%
and 90% of 100% is 100% × 0.9
and so he earns after tax
160 × 0.9 = $144