Answer:
£67.78
Explanation:
Given that rolls come in a package of 20 for £2.87 and ham slices come in a package of 30 for £6.32, you want the minimum cost of enough packs for more than 90 sandwiches, each of which uses 1 roll and 2 ham slices.
Ratios
One package of 20 bread rolls is enough for 20 sandwiches. One package of 30 ham slices is enough for 15 sandwiches. The least common multiple of these numbers is the number of sandwiches that will use a whole number of each of the kinds of packages:
LCM(20, 15) = 60 = 3·20 = 4·15
Packages
We want to make a number of sandwiches that is more than 90. The least multiple of 60 that is more than 90 is 120.
120 sandwiches will require 120/20 = 6 packages of bread rolls, and 120/15 = 8 packages of ham slices.
Cost
The cost of 6 packages of bread rolls and 8 packages of ham slices is ...
6×£2.87 +8×£6.32 = £17.22 +50.56 = £67.78
The least Tina can spend on packs of bread and ham is £67.78.