Answer: she needs to use 4.3 pound of heart shaped chocolate and 8.7 pound of rose shaped chocolate.
Step-by-step explanation:
Let x represent the number of pounds of heart shaped chocolate that she needs to use.
Let y represent the number of pounds of rose shaped chocolate that she needs to use.
She needs to have 13 pounds that sell for 7$ a pound. This means that
x + y = 13
The total cost of 13 pounds of the mixture would be
13 × 7 = $91
She has two types available, a heart shaped chocolate that sells for 4$ a pound and rose shaped chocolate that sells for 8.50$ a pound. This means that
4x + 8.5y = 91 - - - - - - - - - - - - - -1
Substituting x = 13 - y into equation 1, it becomes
4(13 - y) + 8.5y = 91
52 - 4y + 8.5y = 91
- 4y + 8.5y = 91 - 52
4.5y = 39
y = 39/4.5 = 8.7 pounds
x = 13 - y = 13 - 8.7
x = 4.3