Given the expression: S = $\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\frac{1}{15}+...+\frac{1}{20}$

Compare S and $\frac{1}{2}$? ( Fill the suitable sign : > ; < ; = )

Given the fraction : M = $\frac{1+2+...+9}{11+12+...+19}$ . To the value of the fraction is no change, What are numbers deleted in the numerator and the denominator ?

A is 2004 – digit natural number. A divide by 9 ; B is the sum of all digits of A; C is the sum of all digits of B ; D is the sum of all digits of C. Find D ?

Find the natural number such that take $\frac{1}{3}$ of that number divides by $\frac{1}{17}$ of that number, the remainder is 100 ? )

Find the ones digits of the following product  : $2\times 2\times 2\times ...\times 2\times 2\times 2$

( there are 23 factors 2 )

Given 7 fractions : $\frac{7}{5};\frac{4}{13};\frac{2}{13};\frac{1}{18};\frac{7}{3};\frac{4}{5};\frac{7}{4}$ . Nam chooses two fractions that the sum of them is greatest value . Long chooses two fractions that the sum of them is smallest value. Find the sum of four fractions that Nam and Long choose ? ( Write the result as a/b )

Mary has some of marbles that is less than 80 marbles,such that the number of red marbles is 5 times the number of green marbles. If Mary has 3 marbles more , the number of red marbles is 4 times the number of green marbles. How many green marbles does Mary have at first ?

Two people finish a job in 12 hours. The first person needs 10 hours to finish 2/3 the job. How many hours does the second person need to finish the other 1/3?

Two people finish a job in 48 working days. Person A takes a break after 63 days of working, while person B continues to work in the next 28 days, then they finish the job. How long does person A need to finish the job by himself?

Two worker finish a job in 16 hourss. If the first person works in 3 hours, the second person works in 6 hours then they both finish ¼ the job. How long does each worker need to finish the job?

Two workers paint doors on a house in 2 days. The first person works in 4 days then take a break while the second person continues to paint in 1 then they fisish. How many days does each worker need to fisnish the job working seperately?

Two groups of workers do the same job. The first group spends 15 days to finish the job while it takes the second group 12 days. How many days do ½ of the first group and 1/5 of the second group need to finish the job?

Two workers finish a job after 16 hours working together. The first person works in 3 hours, while the second person only finishes 25% of the job. How many hours does the each person needs to finish the job working seperately?

Hoang finishes a job after 10 days working alone, while it takes Minh 15 days. Anh takes 5 times the number of days that both Hoang and Minh need to finish the job. How many hours does the second person need to finish the job working alone?

Three workers do the same job. The first person finishes it in 8 hours working alone. The third person finishes the job after 6 hours and 3 hours for the second person. How many hours does the second person need to finish the job working alone?