Mr. Sparrow was a well-known bird breeder. He had more than 50 and fewer than 100 birds in total. Canaries made up a quarter and finches a ninth of the total number.

How many birds did Mr. Sparrow keep?

(L. Hozová)

Hint. Could he have kept, for example, ninety?
The number of birds kept by Mr. Sparrow had to be divisible by nine (according to the larks) and simultaneously by four (according to the canaries). Such numbers are 36, 72, 108, etc., i.e., multiples of 36. Of these numbers, only $72 \mathrm{v}$ is in the given range.

Mr. Sparrow kept 72 birds.

Note. Alternatively, we can write that the larks were $\frac{1}{9}$ and the canaries were $\frac{1}{4}$ of all the birds. Together, the larks and canaries thus made up $\frac{1}{9}+\frac{1}{4}=\frac{13}{36}$ of all the birds. This ratio can be expressed as

$$
\frac{13}{36}=\frac{26}{72}=\frac{39}{108}=\cdots
$$

The only suitable option is the second one. For completeness, let's add that there were 8 larks and 18 canaries.