algebra

Question

algebra

anonymous · Answer

attachment

mathstudent55 · Answer

Use a proportion.

$\dfrac{y}{x} = \dfrac{y}{x} $

$\dfrac{12}{15} = \dfrac{21}{x} $

Solve for x.
You can start by cross multiplying.

welshfella · Answer

cross multiplying

12 * x = 15* 21
12x =  315

now divide both sides by 12 and you have your answer

tkhunny · Answer

Why not $\dfrac{15}{12} = \dfrac{x}{21}$?

mathmale · Answer

Are you learning from previous practice on this kind of problem?
Remember, if y is directly proportional to x, then y = kx, where k is a constant of proportionality.  This is quite important to know.

tkhunny · Answer

Also, forget you ever heard the term "cross multiply".

$\dfrac{15}{12} = \dfrac{x}{21}$

Factor of 3 on the left.

$\dfrac{5}{4} = \dfrac{x}{21}$

Make your life easier, not harder.

precal · Answer

@tkhunny
I never realized you could factor a number out but it makes sense

mathstudent55 · Answer

@precal
It's not just factoring out a number. It's simply reducing the left fraction.

tkhunny · Answer

Keep your mind open.  $\dfrac{15}{12} = \dfrac{x}{21} \implies \dfrac{15}{4} = \dfrac{x}{7}$

There may be several ways to go about it.