1. World per capita water resources: 2300÷25%=9200 (m3).

Common multiple of 2. 2 3 4 =3*4= 12

So the three of them can meet again in 12 days, that is, 13 years.

3. The middle part =24÷3=8 (m)

The last paragraph =8-2=6 (m)

4. Students participating in this math contest =140 ÷ (116+1/8+25/100) = 320 (people).