Math application problem! Urgent. . .

2,800÷( 1-80%/2)=800÷( 1-40%)≈ 1333,

According to "Xiao Ming ran 80% of the whole course when his younger brother ran the whole course", Xiao Ming ran 40% of the whole course and the remaining 60% when his younger brother ran to the midpoint.

Suppose you want to add x kilograms of water. 40x 10% = (40+x) x5% x = 40kg。

4,

5. Bought watermelon flowers 10X0.7=7 yuan.

Three watermelons, each weighing 4 kg, 3.5 kg and 2.5 kg.

1. If 4 kg of watermelon is not delicious and I only eat 3.5+2.5=6 kg of watermelon, then each kg of watermelon is 7/6= 1. 17 yuan, and I am losing money.

2. If 3.5 kg of watermelon is not delicious, then I only ate 4+2.5=6.5 kg of watermelon, which is 7/6.5= 1.08 yuan per kg of watermelon, and I lost.

3. If 2.5 kg of watermelon is not delicious, and I only eat 3.5+4=7.5 kg of watermelon, then each kg of watermelon is 7/7.5=0.93 yuan, and I have no loss.