need help! do not understand this problem

Question

A phone company offers 2 phone plans. Plan A charges 0.15$ per minute of usage along with a fixed cost of 42$. Plan be charges .20$ per minute of usage with no fixed cost. Anne finds out she could have saved 28$ by switching to Plan B. Find out her minute usage for the month.

Answer 1

0.15 X + 42 - 0.2x = 28

-0.05x + 42 = 28

-0.05x = -14

5x = 1400

x = 280

She used 280 minutes, which under plan A costs her 42 + 0.15*280 = 84.

But under plan B, it would have cost her 0.2*280 = 56, which is $28 less