you knew that it had to be a total of $60.60 and that the flat cost of the plan is $48. so you subtracted the flat cost from the total cost to get the amount that could be spent on gigabytes of data. since you knew that the gigabytes cost $3 each, you divided the remaining budget (12.60) by $3 and got the number of gigabytes that she can get.
sorry if that's confusing or wordy! hope it helps!
