Given that the five friens paid $3.90 each, it means that the total amount that they paid is
3.9 * 5 = $19.5
Let x represent the cost of the chocolate chip cookie cake. Given that the cost includes the price of the cookie cake and $2.50 for extra sprinkles, it means that
x + 2.50 = 19.5
x = 19.5 - 2.5
x = 17
The cost of the chocolate chip cookie cake is $17