First is to find out which price is lower for each when it's only for one ounce.
1) 3.99 / 25 = x / 1
Cross multiply.
3.99 = 25x
Divide both sides by 25.
0.1596 = x
Round to the nearest cent.
$0.16 per ounce
2) 6.99 / 90 = x / 1
Cross multiply the fractions.
6.99 = 90x
Divide both sides by 90.
0.0776667 = x
Round to the nearest cent.
$0.08 per ounce.
6.99 for 90oz is better because it is cheaper by 8 cents per ounce.
