Tell me all equations of percentage like

what is 5% of 80
number should be all the same . like formula.

and tell me how to do it for one by one
like for its 12/60 × 100%

The formula i learned is          is/of = %/100
 
which in your case is      x/80 = 5/100

cross multiply x times 100 and 80 times 5 
 
which would lead to 100x = 400   divide 100 on both sides

400/100            x=4

Ithink you’re answering is b