Answer:
divide the cost by the amount.
4.95/5 = 0.99
8.50/10 = 0.85 (better buy)
3.59/3 = 1.20
4.99/5 = 1.00 (better buy)
1.25/16 = 0.078125
1.55/20 = 0.0775 (this one is the better buy however when you round to 2dp they are both the same price of 0.08.)