Suppose that y varies directly with x and y=10 when x=20. What is y when x=15

y=kx

subst y=10 and x=20 into the above

10=k20

k=10/20

k=1/2

therefore relationship: y=1/2x

subst x=15 into the relationship

y=1/2 (15)

y=7,5

Step by step explanation:

Step 1: when they say y varies directly with x they mean y is proportional to x step 2: so y=kx where k is the constant step 3: is to substitute y=10 and x=20 into the above equation y=kx step 4: you will end up with 10=k20 then divide both sides by 20 so that k becomes the subject of the formula step 5: your answer from the above will be k=10/20 so the relationship is y is directly proportional to 1/2 x what you did here is that you substituted k for 1/2 in the equation in step 3 step 6: is to finally substitute x=15 into the equation y=1/2x to finally get your answer y=7,5.