Part (A)
f = number of fries
c = number of cheeseburgers
1.59f = cost of buying the fries only
3.29c = cost of buying the burgers only
1.59f+3.29c = total cost of burgers and fries
Answer: 1.59f+3.29c
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Part (B)
You buy 3 orders of fries, so f = 3. We'll use the expression we found back in part (A), but we'll tack on an extra 1.99 since you bought the drink for this amount.
So we go from
1.59f+3.29c
to
1.59(3)+3.29c+1.99
after replacing f with 3 and adding on the extra 1.99
Simplify that expression to get the following
1.59(3)+3.29c+1.99
4.77+3.29c+1.99
3.29c+(4.77+1.99)
3.29c+6.76
That expression represents the amount you spend if you buy 3 orders of fries plus a drink of $1.99, on top of some unknown amount of cheeseburgers.
Set that expression equal to 20 and solve for c
3.29c+6.76 = 20
3.29c = 20-6.76
3.29c = 13.24
c = (13.24)/(3.29)
c = 4.024 approximately
Round down to the nearest whole number to get c = 4.
If you buy 4 cheeseburgers, then the total amount spent is 3.29*c+6.76 = 3.29*4+6.76 = 19.92 dollars. The amount you get in change is 8 cents.
Answer: 4 cheeseburgers
