Answer:
Inequality: 4g + 70 < 90
Explanation:
Given that: 
Flat cost per month = $70 
Cost per gigabyte = $4 
Budget amount = under $90 
As she wants to keep the budget under 90, then we will use less than sign to form the inequality.
Let, 
g be the number of gigabytes.
4g + 70 < 90 
Hence, 
Inequality: 4g + 70 < 90