In this case, we'll have to carry out several steps to find the solution.
Step 01:
Data
AT&T $389.99 ===> $15.00 discount
T-Mobile $400.99 ====> $10 mail-in rebate
Sprint $379.00 ===> 10% discount
Straight Talk $400 ===> 20% discount
Step 02:
Cost
AT&T
389.99 - 389.99*0.15 = 331.49
T-Mobile
400.99 - 10 = 390.99
Sprint
379.00 - 379.00*0.1 = 341.1
Straight Talk
400 - 400*0.2 = 320
The answer is:
Straight Talk it is the lowest cost = $320