Inverse Proportion TYS Question 'N' Level

Tmarcus SG

I was doing some past year papers when I came across this question, never been able to do such questions ever since Sec 2, >.<

When some oil floats on water, its upper surface is a circle.

The thickness, t, of the oil is inversely proportional to the square of the radius, r, of the circle. When t = 18, r = 200.

Find t when r = 300.

I know the formula for inverse proportion is y=k/x but I have no idea how to see which is y and which is x, and how to derive k, the constant. Any math pro can help me?

syncopation_music SG

the first variable is the y while the second is the x .

t = K / r ^2

18 = K /200^2

18= K/4000

K = 20000

t = 20000/r^2

t = 20000/(300*300)

t=0.22metres

syncopation_music SG

you got it tmarcus ?

which other question you dont understand ? practice more , thanks