Answer:
500
Step-by-step explanation:
let's say charlie sends x messages per month
he spends $50 + 0.05 for each x (message), which comes out to less than or equal to $75.
as an equation, this is
50 + 0.05 x ≤ 75
subtract both sides by 50 to isolate the variable and its coefficient
0.05 x ≤ 25
multiply both sides by 1/0.05 to isolate x
x ≤ 500
the maximum is therefore x = 500 messages
