Answer:
60 packets
Explanation:
Given that :
Unit cost of 20 packets
$5.49 / 20 =$0.2745
Unit cost of 60 packets :
$10.97 / 60 = $0.1828333
Unit cost of 90 packets :
$18.95 / 90 = $0.2105555
The 60 packets has the least cost per unit, hence it is the best buy.