If x=27 when y=90 and x varies directly with y, then find x when y=80

Answer 1

Note that "x varies directly with y" creates a function:

x = f(y) = ky [where k is a constant; usually, this would be 'y varies directly with x']

What is k? [the constant of variation]

27 = k*(90)

k = 27/90

k = 3/10

Now, given any y, you may find the corresponding value of x:

x = (3/10)*y

x = (3/10)*80

x = 24

Answer 2

27 / 90 = x / 80

Cross multiply.

27 * 80 = 90 * x

2160 = 90x - divide by 90

x = 24