Answer:
Grandmother should purchase 2 units of both products
Explanation:
Let's denote
MU = Marginal utility
P = Price
Q = Quantity
The Utility will be maximum when both unit's MU/P is equal
Product A
Q MU P MU/P
1 80 $5 16
2 40 $5 8
3 30 $5 6
4 15 $5 3
5 7 $5 1.4
6 6 $5 1.2
7 5 $5 1
8 3 $5 0.6
Product B
Q MU P MU/P
1 30 $2 15
2 16 $2 8
3 15 $2 7.5
4 14 $2 7
5 13 $2 6.5
6 12 $2 6
7 11 $2 5.5
8 10 $2 5
As you can see above MU/P is same when grandma purchase 2 product each to maximize utility.