There are a couple of ways to look at this.
1) Set up a ratio: 2 cups sugar to 5 cups flour = 5 cups sugar to x cups flour
2/5 = 5/x
cross multiply
2x = 25
x = 12 1/2 cups flour
2) Look at the factor by which the cups of sugar was increased and increase
the cups of flour by the same factor.
2x = 5
x = 2.5
The sugar was increased by a factor of 2.5 so increase flour by same factor.
5(2.5) = 12 1/2 cups flour