Answer:
The number of single crust apple pies sold are 16 and the number of double crust apple pies are 20 .
Explanation:
Let us assume that the numbers of single crust apple pies are x .
Let us assume that the numbers of double crust apple pies are y .
As given
Grandmas bakery sells single crust apple pies for $6.99 and double crust apple pies for $10.99.
The total number of pies sold on a busy Friday was 36 if the amount collected for all the pies that day was $331.64 .
Equations becomes
x + y = 36
6.99x + 10.99y = 331.64
Simplify the above equation

699x + 1099y = 33164
Two equations are
x + y = 36
699x + 1099y = 33164
Multiply x + y = 36 by 699 and subtrcted from 699x + 1099y = 33164 .
699x - 699x + 1099y - 699y = 33164 - 25164
400y = 8000

y = 20
Putting all the values in the formula
x + 20 = 36
x = 36 -20
x =16
Therefore the number of single crust apple pies sold are 16 and the number of double crust apple pies are 20 .