Shelf A had 40 books and Shelf B had 16 books. After Katrina borrowed an equal number of books from Shelf A and Shelf B, there were 3 times as many books on Shelf A than Shelf B. How many books did Katrina borrow altogether?

8 books; 4 from each shelf

Step-by-step explanation:

let x represent equal number of books from each shelf

from shelf A = 40 - x

from shelf B = 16 - x

shelf A has 3 times as many books than shelf B

3 (16 - x) = 40 - x

48 - 3x = 40 - x

48 - 40 = - x + 3x

8 = 2x

x = 4

altogether she borrowed 4 + 4 = 8 books