Is y=7.5x a proportional relationship and why

Direct Proportion functions look like this:

y=kx

'k' is the constant of proportionality and in this case its 7.5 or 15/2.



When x=1, y=7.5
When x=2, y=15
When x=3, y=22.5

Therefore y is directly proportional to x.



DIRECT PROPORTION EXPLAINED:

Say a football costs £7.50. If you buy one football you'll have to pay out £7.50, but if you you buy two footballs you'll have to pay out £15.00. Therefore the cost of the football(s) is directly proportional to the amount of footballs you buy. C=cost, f=football(s) and C∝f, therefore C=kf, but as k=7.5, C=7.5f.

INVERSE PROPORTION EXPLAINED:

If it were to take 8 hours for one bricklayer to set up a wall, how long would it take for two bricklayers to set up a wall? The answer in this case would be 4 hours.

T=time to set up a wall
b=bricklayer(s)

Therefore T∝1/b, and T=k/b. In this case k=8 so T=8/b.

When b=1, T=8.
When b=2, T=4.

We'd say that the time it would take for bricklayers to set up a wall would be inversely proportional to the amount of bricklayers available.