Question

The value of y varies directly with x, and y=3 when x=9 find y when x = 21

Answer 1

Answer: y = 7

Step-by-step explanation:

Step 1) plug in 3 for y and 9 for x in the direct variation formula, which is y = kx

3 = k(9)

Step 2) Solve for k

3 = k(9) (Divide by 9 on both sides)

3/9 = k (Now simplify 3/9)

1/3 = k. So k = 1/3

*just FYI: “k” is the constant of variation.

Step 3) use the equation y = kx and plug in 1/3 for k

y = 1/3x

Step 4) using the equation y = 1/3x, plug in 21 for x.

y = 1/3(21) (1/3*21 is the same as 21/3)

y = 21/3 (21/3 = 7)

y = 7. So that’s you’re answer! When x=21, y=7 because 7 is 1/3 of 21!

Hope this helps! My best wishes to you as you go through school!