I guess 3 and 19? Or are you asking a different question?
S1=$70/20gal=$3.50 per gallon
s2=$33.50/(50/4)gal=$2.68 per gallon
s3=$48.75/(15+8)gal=$2.12 per gallon
So supplier #3 has the best deal.
From order of operations, we do multiply/divide first, then add/subtract. So:
(I assume that is 8 times 2, and 5 times x, solving for x)
6+8<span>⋅</span>2 = 5x
6 + 16 = 5x
22 = 5x
x = 22/5 (twenty two fifths)