"Divide b into a" means the same as "divide a by b".
As an example, a person can easily read "divide 24 into 6" as "divide 24 into 6 [parts]," i.e. 24/x = 6, x = 4 — as opposed to 6/24 = x, x = 0.25. This use is especially reinforced by everyday experiences like dividing a cake into eight slices.