Answer:
$40 + 10x
Explanation:
Given that:
Original cost of data plan = $40 per month for 10 GB of data
Additional data cost = $10 per GB when usage is above the original data cap
Hence, amount to be paid when usage is over the original data cap by x GB :
Original amount + (additional cost per GB * additional GB used)
$40 + ($10 * x)
$40 + 10x