20. Schoolchildren helped the collective farm workers to harvest vegetables. The fourth grade collected 18 tons of cabbage and as much carrots as the sixth grade collected onions. The fifth grade worked only on cucumber harvesting and collected a whole number of tons of cucumbers, which is less than the amount of cucumbers collected by the sixth grade but more than the amount of carrots collected by the fourth grade. The sixth grade collected 7 tons of onions, and cucumbers - half as much as the cabbage collected by the fourth grade. How many tons of vegetables did the schoolchildren collect in total?
20. The sixth grade collected 9 centners of cucumbers $(18: 2=9)$ and 7 centners of onions, which means the fourth grade collected 7 centners of carrots. The fifth grade collected fewer cucumbers than 9 centners but more than 7 centners, so 8 centners. Thus, the students collected in total $18+7+8+9+7=49$ (centners).