To keep her bill at $71 per month, she needs to consume 4.2 gigabyte in a month.
A linear equation is in the form:
y = mx + b;
where y, x are variables, m is the slope of the line and b is the initial value of y.
Let y represent the amount of money paid for x gigabyte.
Given a flat cost of $50 per month and $5 per gigabyte, hence m = 5, b = 50. Therefore:
y = 5x + 50
For a bill of $71 per month:
71 = 5x + 50
5x = 21
x = 4.2
To keep her bill at $71 per month, she needs to consume 4.2 gigabyte in a month.
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