Question

Ann received a $90 gift card for a coffee store. She used it in buying some coffee that cost $7.82 per pound. After buying the coffee, she had $66.54 left on her card. How many pounds of coffee did she buy?

Answer 1

3 lbs of coffee

1) We can solve this problem, by setting an equation at first:

$66.54=90-7.82x$

Note that, in this equation: 90 is the initial value, and x is the number of pounds of coffee. And also, the more Ann buys the less money she gets that's why the sign of 7.82 is negative.

2) Let's solve it for x:

$\begin{gathered} 66.54=90-7.82x \\ \\ 90-7.82x=66.54\:\:\:Let^{\prime}s\:work\:with\:integer\:numbers\:multiplying\:by\:100 \\ \\ 90\cdot \:100-7.82x\cdot \:100=66.54\cdot \:100 \\ \\ 9000-782x=6654 \\ \\ 9000-782x-9000=6654-9000 \\ \\ -782x=-2346 \\ \\ \frac{-782x}{-782}=\frac{-2346}{-782} \\ \\ x=3 \\ \end{gathered}$

Thus, Ann bought 3 lbs of coffee