Hey there! :)
You have to divide the "total cost" by "number of books" to find the unit price and see which is the least
Jo = 9 ÷ 4 = $2.25/b
Kei = 12 ÷ 8 = $1.50/b
Kate = 30 ÷ 16 = $1.88/b
Bryn = 8 ÷ 4 = $2.00/b
The lowest is Kei with $1.50 per book
Your answer is Kei
Hope this helps :)