Answer:
x=4.33
Explanation:
let he total cost be y
and the number of plaque be x
Company A charges
y=28+2.5x-----------1
Company B charges
y=15+5.5x--------------2
Equating 1 and 2 we can find x
28+2.5x=15+5.5x
collect like terms
28-15=5.5x-2.5x
13=3x
x= 13/3
x=4.33