Hello there! The cost of a cookie is $0.72.
Start by multiplying the cost of lunch by 5 and subtract that from the total to get rid of the cost of lunch.
$1.85 x 5 = $9.25
$20 - $9.25 = $10.75
So, there were FIVE students and each bough THREE cookies. To find the cost for each cookie, first divide the total spent on cookies by 5 to get what each student spent on cookies, then by 3 to see how much each cookie cost.
$10.75/5 = $2.15
$2.15/3 = 0.716666667, which can be rounded to $0.72.
I hope this helps and have a great rest of your day!