Question

12 1/2 of what amount is $10.40?

Answer 1

of = multiply
what = x
is means =

(12.5)x = 10.4
x = 10.4/12.5 = 104/125

Answer 2

Let x = what amount

Rewrite 12 1/2 as 25/2.

(25/2)(x) = 10.40

Take it from here. Solve the equation for x.