Question

How do you set the expression

Answer 1

((X x $25) x $0.06) + ((Y x $4) x $0.06)

Answer 2

25x + 4y is the total cost.

.06(25x+4y) is the total tax.
Rewrite using fractions then expand:

6/100 * (25x + 4y)
(150/100)x + (24/100)y
1.5x + .24y  is the expression to find tax.