Answer
The client would make a call of 400 minutes for the cost of the two plans to be equal
Explanation:
Let C be the cost
Let x be the number of minutes of call
For the first plan
The company charge $0.18 for each minute
Let the total cost
C = 0.18 * x
C = 0.18x -------------- equation 1
The second plan
The second plan has a monthly charge of $20 and an additional $0.13 for each minute call
C= 20 + 0.13 * x
C = 20 + 0.13x -------------------- equation 2
To know the number of minutes that will make the cost of the plan equal, then, we need to combine the two equations
0.18x = 20 + 0.13x
Subtract 0.13x from both sides
0.18x - 0.13x = 20 + 0.13x - 0.13x
0.05x = 20
Isolate x by dividing through by 0.05
0.05x / 0.05 = 20/0.05
x = 400 minutes
Hence, the client would make a call of 400 minutes for the cost of the two plans to be equal