Question

When x=8,y=20.find y when x=20

Answer 1

When x=8,y=20. Find y when x=42

As Andy said, there are many ways of looking at this:

Setting up a proportion gives us...
8 is to 20 as 42 is to y

8/20 = 42/y
8y = 20(42)
y= 20(42)/8
y = ____

If this is a direct relation:
y = kx
20 = 8k
k = 20/8
k = 2.5
y = 2.5x

y = 2.5(42)
y = _____

Note: This should be the same answer as the proportion gave.

Then of course, there is the question of whether you are trying to
find a geometric or arithmetic series or sequence with a pattern ....
Thus, Andy C's response.

Answer 2

20/8 = 2.5

20 * 2.5 = 50

x = 8 * 2.5

y = 20 * 2.5

When x = 20, y = 50