Suppose these three watermelons have a, b and c kilograms respectively.

Then a =10 * 8/20 = 4kgb =10 * 7/20 = 3.5g.

C = 10 * 5/20 = 2.5kg。

Scheme 1. Raw food is a kind of food.

The actual unit price is 0.7 *10/(3.5+2.5) = 7/6 >; 1 loss.

Option 2. B it's not ripe and can't be eaten.

The actual unit price is 0.7 *10/(4+2.5) = 7/6.5 >; 1

suffer wastage

Option 3. C is not ripe and can't be eaten.

The actual unit price is 0.7 * 10/(4+3.5) = 7/7.5.

Don't suffer

2. 1 min The water discharged from the water pipe is 33.9 cubic meters.

6dm=0.6m

3.14 * (0.6 \ 2) * (0.6 \ 2) * 2 * 60 = 33.912 ≈ 33.9 (square meter)

3. So the most economical plan is to buy 50 copies and give them to 240 yuan.

You can enjoy a 20% discount above 300 yuan. You need to buy 300/6=50 copies and spend 300*0.8=240 yuan.

Enjoy a 10% discount above 50 yuan, and the total cost is 45 * * * 0.9 = 243 yuan > 240 yuan.

So the most economical plan is to buy 50 copies and give them to 240 yuan.

This score is 5/8.

( 1*4)/(2*4)=4/8

(4+ 1)/8=5/8

5/8+3/8=8/8= 1

So the original score is 5/8.

5.

Solution: Suppose it is actually completed in X days:

x:30 = 1500/30:( 1500/30+20)

x:30=50:70

x=50×30÷70

= 150/7

About 22 days.

6. Its area is about 1256 square meters.

R=20, S=πr squared =400π.

It is approximately equal to 1256 square meters.

7. There are 30 boys and 32 girls.

Boys go 1/5, and the rest 4/5. There are 8 girls, and the rest are 62-8- boys. Let the number of boys be x, and the formula is: 4/5*X=62-8-X, and the number of boys is 30, so the number of girls is 32.

8. Team A deserves 3300 yuan.

A completes 1/4 first, leaving 3/4.

3/4 is completed by three teams * * * It takes 6 days to complete, so divide by 3/4 (1/20+1/24+1/30).

So a * * * has done 1 1 day, 6000/20* 1 1 is enough.

9. Loss 162 (RMB)

Original price: 819/(1-30%)+819/(1+30%) =1800 > 2 * 819,

So the loss is1800-2 * 819 =162 (yuan).

10. Bookstore * * * sold 205 calendars.

Solution: Assuming that the original book A is sold and the book B is sold after the price reduction, the equations are obtained:

18A+8B=2870

B=2/3A

Solution, 18A+8*2/3A=2870.

A= 123

B=82

A+B=205

1 1. This oil drum can hold 75.36 liters of gasoline at most.

The bottom radius of the barrel is 6* 1/3=2 decimeters.

So the volume of the barrel is ∏ * 2 2 * 6.

It is approximately equal to 3. 14*4*6=75.36 cubic decimeter.

Since 1 decimeter cubic = 1 liter, the answer is 75.36 liters.

12. It costs 5.25 million to make a suit for children.

The original fabric is regarded as the unit "1"

Adults 1 suit 1/6, children 1 suit 1/8.

The original cloth length is 7/4 ÷ (1/6-1/8) = 42m.

1 children's clothing 42×1/8 = 5.25m.

13. The distance between the two cities is160km.

Suppose two cities are x kilometers apart.

6*4=24

1/2x+ 16=0.75x-24

40= 1/4x x= 160

14. This rope is 255 feet long.

Suppose it takes x feet to go around the tree, then

4x+305=6x-205

X=255 feet.

15.

Original price:1800/(1-25%)+1800/(1+25%) = 3840 > 2 *1800,

So the loss is 3840-2* 1800=240 yuan.