If y varies directly as x, and y = 30 when x = 4, how do you find the constant of proportionality?

1 Answer
May 31, 2016

It is #7.5#.

Explanation:

if it varies directly means that #y# is equal #x# multiplied by something. I call this "something" #A#.
So I write

#y=Ax#

To find the value of #A# I have to divide on both sides for #x# obtaining

#y/x=Ax/x#

the two #x# on the right cancel each other and I remain with

#y/x=A#.

Now I can substitute the numbers

#30/4=A#
#7.5=A#.

Then the proportionality constant is #7.5#.