4/7 can’t divide into 3 units nicely. So let’s look at it as 12/21 instead.
We can use units and parts to solve this from here,
Let what Kim had at first be 21u
3 books = 12u, then each book will be 4u.
Of the remaining money= 21u – 12u = 9u
10 markers = 9u – 4u = 5u
a) one marker = 5u/10 = 0.5u
She could have bought = 21u/0.5u = 42 markers##
b) 4u = $10.80
u = 10.80/4 = $2.70
Marker = 0.5u = 2.7/2 = $1.35#