Answer:
Total cost when over by 3 GB: $90
Total cost when over by x GB: $30x
Step-by-step explanation:
"will charge an extra $30 per GB over the included amount"
For 1 GB over the limit it charges $30.
For 3 GB, it charges 3 × $30 = $90
For X GB, it charges x × $30 = $30x