Answer :

10% is $3.00
5% is $1.50 
1% is $0.30
3% is $0.90
% means out of 100 so 1%=1/100

so 3%=3/100
$30.00=3000 cents
so you multiply 3/100 by 3000 =(3*3000)/100
you can cross out two zeros at the bottom and on the top and get
(3*30)/1 or 90/1 or 90 cents

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