13.061. To pay for the delivery of four parcels, 4 different postage stamps were needed for a total of 84 kopecks. Determine the cost of the stamps purchased by the sender, if these costs form an arithmetic progression, and the most expensive stamp is 2.5 times more expensive than the cheapest one.
Solution.

Let $x$ kop. - the cost of the cheapest stamp, $x+d, x+2 d$, $x+3 d$ - the costs of the other stamps. By condition

$$
\left\{\begin{array}{l}
x+x+d+x+2 d+x+3 d=84, \\
x+3 d=2.5 x,
\end{array} \text { from which } x=12 ; d=6\right. \text {. Costs of the }
$$

stamps: 12 kop., 18 kop., 24 kop, 30 kop.

Answer: 12 kop., 18 kop., 24 kop, 30 kop.