What’s the answer for this question?

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!