"A ratio of 4 to 5" means that dividing the two angles (whatever they may be) simplifies to 4/5. One example is 40 degrees and 50 degrees so 40/50 = 4/5. Another example is the pair of angles 20 and 25, so 20/25 = 4/5
For now, we don't know the angles. Let's call them x and y. Based on the info above, we can say
x/y = 4/5
also we know that x and y add to 90 degrees because the angles are complementary. So x+y = 90 which solves to y = 90-x if you isolate y.
Let's cross multiply on the first equation to get...
x/y = 4/5
5*x = 4*y
Now replace y with 90-x. Solve for x
5x = 4(90-x)
5x = 360 - 4x
5x+4x = 360
9x = 360
x = 360/9
x = 40
Use this to find y
y = 90 - x = 90 - 40 = 50
So the two angles are 40 degrees and 50 degrees.
