X + 4Y = 20.
You cannot find both x and y as an exact number. You can find x in terms of y and y in terms of x though.
subtracting 4y from both sides gives χ= 20-4y
subtracting x from both sides of the original equationgives
4y = 20 -x
dividing by 4 gives y= 5 - (x/4)
if you graph this function, (just google it) you will see that for each value of y, x is different.