$120 for 20 lessons
Step-by-step explanation:
Divide each to find the cost for each lesson:
First choice: 90/10 = 9 or $9 per lesson
Second choice: 160/20 = 8 or $8 per lesson
Third choice: 80/10 = 8 or $8 per lesson
Fourth choice: 120/20 = 6 or $6 per lesson.
$120 for 20 lessons is your answer for it is cheaper by $2 (in comparison to choice 2 & 3), and by $3 (in comparison to first choice).