First, you would need to determine how much it would cost her if it was three for $1 or seven for 42.
Three for $1:
21 ÷ 3 = 7
7 × $1.00 = $7.00
Seven for $2:
21 ÷ 7 = 3
3 × $2.00 = $6.00
Now, all you would need to do is find the difference.
$7.00 - $6.00 = $1.00
Jackie would have saved A. $1.00.
I hope this helps!