If x% of y is equal to z, then what percentage of z is x?

If x% of y is equal to z

$$\dfrac{x}{100}\times y=z$$ -----equation (1) (Eg: 40% of 120 = $$\dfrac{40}{100}\times120$$)

We need to find what percentage of z is x

$$\dfrac{x}{z}\times100$$ = ?  ------equation (2)

$$\dfrac{x}{100}\times y=z$$

We can write this as $$\dfrac{x}{z}=\dfrac{100}{y}$$   ---equation (3)

Substituting the value of equation (3) in equation (2)

$$\dfrac{x}{z}\times100$$ = $$\dfrac{100}{y}\times\ 100\ =\dfrac{100^2}{y}$$