$18 + $.9(5%) = $18.9
the thing with percentage is just moving decimals 100% = 1, 45% = .45, 10% = .1 and so on
so here you can solve it by taking 18 and multiplying it by .05(which is 5% if you understand my examples above)
.
or orally like i did, (note 10% is moving the decimal to the left one time so 10% of 18 is 1.8 and half of 1.8 is .9 which is 5% since the half of 10% is 5% lol )