X-20=280 X+30=330
There are 280 trees in grade four, 300 trees in grade five and 330 trees in grade six.
2. Let the passenger ship speed be V 1 and the cargo ship speed be V2.
Then the distance between the two ports is (V 1+V2)× 10.
(v 1+V2)×4 =(v 1+V2)× 10-(250+350)
(V 1+V2) = 100 Distance between two ports = 1000 km.
3. Set the length of this water meter to x meters.
(X-200-2/5X)=(200+2/5X)/2
X = 750km km
4. The original number of female students is 36× 4/9 = 16.
Besides, there are x girls.
16+X/36+X=9/ 19
X=2, and then two girls came.
5. Category A donations are 1 of the other three categories.
Then the number of donations for Class A = 3960× 1/3 = 1320.
The number of class B donations = 3960× 1/4 = 990.
The number of donations in category C = 3960× 1/5 = 792.
Number of donors in Ding Ban =3960- 1320-990-792=858.
6.A+B = 100 B+C = 1 15 C+A = 95
(2)-( 1) gives C-A = 15 C =55.
a = 95-55 = 40 B = 1 15-55 = 60
A 40 kg, B 60 kg, C 55 kg.
7.400 ÷ (100-80) = 20 minutes
Meet every 20 minutes.
8.50 km/h =500/36 m/s
(120+880) 500/36 = 72 seconds
It takes 72 seconds for the train to get on the bridge and leave completely.
9. Let this book have x pages.
The ratio of read to unread is 2: 7.
Read is 2/9X, unread is 7/9X.
2/9X+44:7/9X-44=3:5
X=288
This book has 288 pages.
10. 15:24=5:8
15- 10 = 5 The price of five Chinese books is the same as that of eight math books.
The rest of the money is spent on math books, or you can buy eight math books.