This problem calls for calculating two different unit prices:
3 bars
--------- = $0.33 (or $0.34) per bar; 21 bars would cost $6.93.
$1
7 bars
--------- = $0.35 per bar; 21 bars would cost $7.35
$2
By purchasing the bars at the cheaper price Jackie could have saved
$7.35 - $6.93, or $0.42 (answer)