divide 18 cups if flour into two groups so the ratio is 4 to 5

Let us divide the flour into two groups, A and B

now we know from the given question

A/B = 4/5

A = 4B/5

We also know there are 18 cups of flour in total.

A + B = 18

So

(4B/5) + B = 18

(4B/5) + (5B/5) = 18

9B/5 = 18

B = (18*5) / 9

= 10

Then

A = 18 - B

= 18 - 10

= 8

So the ratio of dividing the 18 cups is 8:10