A scooter's petrol tank was $\frac{4}{5}$ full in the morning. By night it was only $\frac{1}{3}$ full. By what fraction of the tank did the petrol level fall?

A fraction a/b says: the whole is split into b equal parts, and we are talking about a of them. So 3/8 means: divide the whole into 8 equal parts, take 3 of them. The two equal parts is what makes it a fraction — three slices of an unevenly cut cake do not give you 3/8. The denominator must always be a positive whole number. The numerator can be 0, equal to the denominator (giving 1), greater than the denominator, or less than it.