Under her cell phone plan, Sarah pays a flat cost of $69 per month and $4 per gigabyte. She wants to keep her bill at $83.80 per month. Write and solve an equation which can be used to determine gg, the number of gigabytes of data Sarah can use while staying within her budget.

To determine the number of gigabytes \( g \) that Sarah can use while keeping her total bill at or below $83.80, we can set up the following equation based on her cell phone plan:

1. The flat cost per month is $69.
2. The cost per gigabyte is $4.

The total cost \( C \) can be expressed as:

\[ C = 69 + 4g \]

We want to keep this total cost at $83.80, so we set up the equation:

\[ 69 + 4g = 83.80 \]

Now, let's solve for \( g \):

1. Subtract 69 from both sides:

\[ 4g = 83.80 - 69 \] \[ 4g = 14.80 \]

2. Divide both sides by 4:

\[ g = \frac{14.80}{4} \] \[ g = 3.70 \]

Since Sarah can only use whole gigabytes of data, we will round down to the nearest whole number:

\[ g = 3 \]

Thus, Sarah can use 3 gigabytes of data while staying within her budget of $83.80.