Daddy decided to give his son Mojmir a monthly allowance. Mojmir received his first allowance in January. Daddy increased the allowance by 4 Kč every month. If Mojmir didn't spend any, he would have 900 Kč after the twelfth allowance before Christmas. How many Kč did Mojmir receive for his first allowance in January?

(L. Hozová)
Let's denote the amount of Mojmír's pocket money in January in Kč as $x$. In February, Mojmír received $x+4$, in March $x+8$, in April $x+12, \ldots$, in December $x+44$. According to the problem, we know that

$$
12 x+(4+8+12+16+20+24+28+32+36+40+44)=900 .
$$

After simplification, we get:

$$
\begin{aligned}
12 x+264 & =900, \\
12 x & =636, \\
x & =53 .
\end{aligned}
$$

Mojmír received 53 Kč in January.