If y=15 when x=3, find x when y=20.

Question

johnweldon1993 · Answer

Well it looks like as 'x' goes up so does 'y'

Direct Variation
$$\large y = kx$$

So when y = 15 and x = 3

$$\large 15 = 3k$$
$$\large k = 5$$

This is our common ratio...

Now that we have it

$$\large y = 5x$$

we can solve for 'x' in the second part....when y = 20

$$\large 20 = 5x$$
$$\large x = \frac{20}{5}$$
$$\large x = ?$$

unklerhaukus · Answer

But how can you assume a direct variation?

anonymous · Answer

Yes it's Direct Variation as it says in the instructions. I got to y=5x, but how did you get the 20? That's what confuses me, thanks.

johnweldon1993 · Answer

Right...Well as it asks in the question...

"find x when y = 20"

johnweldon1993 · Answer

So where you have

$$\large y = 5x$$

To solve for 'x' all you do is plug in that 'y' they provided you

$$\large 20 = 5x$$

anonymous · Answer

oh haha! Okay wow, thanks I don't know how I didn't see that. Thanks man, you helped me :)

johnweldon1993 · Answer

No problem :)