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## Constant of proportionality

## Video transcript

- [Instructor] We're told the quantities x and y are proportional, and then they give us a
table where they give us a bunch of x's and they give
us the corresponding y's. When x is four, y is 10. When x is five, y is 12.5,
and so on and so forth. Find the constant of proportionality, r, in the equation y is equal to r times x. So pause this video and see
if you can figure that out. All right so we wanna find the constant, the constant of proportionality, I sometimes have trouble saying that, r, where if I for any x, if I multiply it by r I get y. Well we just have to
look at each of these x's and figure out well what
are we multiplying by to get to y. So what do I have to multiply
four by to get to 10? Well if you multiply it by 10/4, if you multiply it by 10/4,
then you're gonna get to 10. And 10/4 is the same thing
as multiplying it by, let's see that is 2 1/2. So this is times 2 1/2. So let's see does that hold? In every case if I multiply by 2 1/2, if I multiply x by 2 1/2, do I get y? So five times 2 1/2, five times two is 10, and
then another 1/2 is 2.5. It indeed does equals 12.5. Five times 2.5. And then here 10 times 2.5 is clearly 25. And you can work it out by hand. I did the little bit of the
multiplication in my head, but you can see in every case, I take my x, I multiply
it by 2.5, I get my y. So in every case I take
my x and multiply it by the constant of
proportionality 2.5, I get my y. So what's the constant of proportionality? It is 2.5.