So,
We have three placeholders.
x, y, and z will represent the number of cookie boxes they sold for the first, second, and third weeks, respectively.
"The girls sold 5 more boxes the second week than they did the first."
x + 5 = y
"They doubled the sales of the second week for the third week."
z = 2y
"To sell a total of 431 boxes of cookies"
x + y + z = 431
Now we have our three equations.
x + 5 = y
z = 2y
x + y + z = 431
Obviously, we can substitute y and z in the last equation, because the other two open sentences tell us what they are.
x + (x + 5) + 2(x + 5) = 431
x + x + 5 + 2(x + 5) = 431
Collect Like Terms.
2x + 5 + 2(x + 5) = 431
Distribute.
2x + 5 + 2x + 10 = 431
Collect Like Terms.
4x + 15 = 431
Subtract 15 from both sides.
4x = 416
Divide both sides by 4.
x = 104
Now we can use the other two original sentences to figure out what y and z are.
x + 5 = y
104 + 5 = y
109 = y
z = 2y
z = 2(109)
z = 218
Now, to check, add x, y, and z.
x + y + z = 431
104 + 109 + 218 = 431
213 + 218 = 431
431 = 431 This checks.
The Girl Scouts sold 104 cookie boxes on the first week, 109 boxes on the second week, and 218 boxes on the third week.