Suppose there are 6X girls and 7X boys.

Understand the theme

(6X+6)/[(6X+6)+7X)=20/4 1

The solution is X=9, so

The original school had 13*9= 1 17 students.

2

When they meet, the distance between trucks is the distance between Party A and Party B. 。

After walking for 3 hours, the ratio of the distance covered to the remaining distance is 2: 3.

If this distance is 5X.

After 3 hours, the distance traveled is 2X.

There are 3X left, and it takes 3/2X*3X=4.5 hours.

three

Set * * * to buy X colored balls? If you have any questions, you know that the red ball is 2/5X and the other balls are 3/5X.

2 5X( 1+ 1/8)= 1-2/5X- 15

The solution is X= 100.

A: I bought 100 colored balls.