I have been trying the IF formula and i am stuck.

EX. if(logic,true,false)got that
a1=1
a2=$20

IF(.01<=a2<=.99,a1*.2,0)andIF(1<=a2<=9.99,a1*.4,0) andIF(10<=a2<=24.99,a1*.60,0).................

i cant figure a formula
help please
plus if a2=1-25 at 5.25%,if a2=25.01-50 than 25 at 5.25%plus 25.01-50 at
3.75%...................
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or a book with diff formulas
Brian