1 đàn vịt có số con là : 

             9816 : 50 = 196,32 ( con )

10 đàn vịt có số con là :

              196,32 x 10 = 1963,2 ( con )

                           đáp số : 1963,2 con

Nhóm 1 : Từ 1 đến 999 

Chữ số 1 được viết số lần là : 3000 : 10 = 300 (lần)

Nhóm 2 : Từ 1000 đến 1999

Chữ số 1 được viết số lần là: 1000 + 300 = 130 (lần)

Nhóm 3 : Từ 2000 đến 2014 chữ số 1 được viết 7 lần.

Vậy từ 1 , 2 , 3 , ... 2014 có số chữ số 1 là : 300 + 1300 + 7 = 1607 (lần)

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 37.

  Humans have struggled against weeds since the beginnings of agriculture. Marring our gardens is one of the milder effects of weeds – any plants that thrive where they are unwanted. They clog waterways, destroy wildlife habitats, and impede farming. Their spread eliminates grazing areas and accounts for one-third of all crop loss. They compete for sunlight, nutrients, and water with useful plants.

The global need for weed control had been answered mainly by the chemical industry. Its herbicides are effective and sometimes necessary, but some pose serious problems, particularly if misused. Toxic compounds threaten animal and public health when they accumulate in food plants, groundwater, and drinking water. They also harm workers who apply them.

  In recent years, the chemical industry has introduced several herbicides that are more ecologically sound. Yet new chemicals alone cannot solve the world’s weed problems. Hence, scientists are exploring the innate weed-killing powers of living organisms, primarily insects and microorganisms.

The biological agents now in use are environmentally benign and are harmless to humans. They can be chosen for their ability to attack selected targets and leave crops and other plants untouched. In contrast, some of the most effective chemicals kill virtually all the plants they come in contact with, sparing only those that are naturally resistant or have been genetically modified for resistance. Furthermore, a number of biological agents can be administered only once, after which no added applications are needed. Chemicals typically must be used several times per growing season.

The word ‘clog’ in bold is closest in meaning to ........

A. drain

B. grow along

C. obstruct

D. float on

a/ Oxit tác dụng với H2O là: N2O5, SiO2

H2O + N2O5--> 2HNO3

H2O + SiO2 --> H2SiO3

b/ Oxit tác dụng với dd H2SO4: Fe2O3, Fe3O4, ZnO, Al2O3

3H2SO4 + Fe2O3---> Fe2(SO4)3 + 3H2O

4H2SO4 + Fe3O4---> FeSO4 + Fe2(SO4)3 + H2O

H2SO4 + ZnO ---> ZnSO4+ H2O

3H2SO4 + Al2O3---> Al2(SO4)3 + 3H2O

c/ Oxit tác dụng với dung dịch NaOH: ZnO, Al2O3

2NaOH+ZnO--->Na2ZnO2 + H2O

2NaOH + Al2O3--> 2NaAlO2 + H2O

1016 nha mình nhầm

bạn xem lại xem có bị nhầm đề bài k z, vì thể tích ở đktc (V=n.22,4) chỉ dùng cho chất khí thôi mà