4. If a schoolboy bought 11 pens, he would have 8 rubles left, but for 15 pens, he would be short of 12 rubles 24 kopecks. How much money did the schoolboy have?
4. 5) $15-11=4$ pens - bought more the second time;
2) $8+12=20$ rubles - amount to four pens;
3) $20: 4=5$ rubles - cost of one pen;
4) $5 \times 11+8=63$ rubles - the schoolboy had.

Answer: the schoolboy had 63 rubles.