The son’s present age is equal to $\frac{1}{2}$ of the difference between dad’s age and age of the son. 4 years ago, the son’s age is equal to $\frac{1}{3}$ of the difference between their ages. If the son’s age if $\frac{1}{4}$of the difference between their ages, how old is each of them?

A store has three oil tanks A,B and C. Tank A is filled while tank B and C are empty. If we poor oil in tank A into tank C, then $\frac{5}{9}$ of the oil remains in tank A. If we use the oil in tank A to fill in tank C and B, then 4 liters of oil will be needed. How many liters each tank can contain? 

A team joined a student contest in three subjects Literature, Math and English, held by the city council. They achieved 15 rewards. How many students are there in the team? Knowing that

Every students has a prize

At least 1 student gets 1 prize in every subject

At least 1 student gets 2 prizes in any two subjects

At least 1 student gets 3 prize in all three subjects.

The total number of students getting 3 awards, 2 awards and 1 ward increases in order.

Find the  four consecutive natural numbers from small to large so that the product of them is 3024 ?

Find the smallest natural number which has a remainder of 1 when it is divided by 2, has a remainder of 1 when it is divided by 5, has a remainder of 3 when it is divided by 7 and divisible by 9?

At the end of the first semester, the Math test in grade 5A, the number of excellent students is equal to 3/7 the total student in the class. In the middle of the second semester, 3 students get good scores in the Math test so the number of excellent students is equal 2/3 to the rest of the class. How many students have good scores in the middle of the second semester? Know that the number of 5A students is unchanged in both two semesters.

The number of marbles that Ti has is no more than 80. Red marbles are 5 times more than blue marbles. If Ti has 3 more blue marbles, then the number of red marbles is 4 times more than blue marbles. How many marbles does Ti has for each colour that the beginning?

7 fractions are given:

$\frac{7}{5};\frac{4}{13};\frac{2}{13};\frac{1}{18};\frac{7}{3};\frac{4}{5};\frac{7}{4}$

Thang has to choose 2 fractions which give the the greatest sum. Long has to choose 2 fractions which give the smallest sum. Find the sum of 4 fractions that Thang and Long choose.

Find the digit a and b so that:

$\frac{1}{3}=\frac{1}{a}+\frac{1}{b}$

There are 5 teams participating in a contest. The total score of 5 teams is 144 points. It is interesting that all 5 teams achieve one in each prize: first prize (30 points), second prize (29 points), third prize (28 points). How many more teams that achieve the third prize than first prize?

Dong had 400 000 dong. He spent $\frac{1}{5}$ the money on a storybook and $\frac{3}{10}$  on a calculator. Hơ many money did he spend altogether ?

Find a natural number which has a remainder of 100 if we take $\frac{1}{3}$of the number divided by $\frac{1}{17}$ of the number ?

Son’s current age is $\frac{1}{2}$ the difference between father’s age and son’s age. Four years ago, the son’s age is $\frac{1}{3}$ the difference between their ages. How old is father’s age when son’s age is $\frac{1}{4}$  the difference of their ages?

Square ABCD is given. Cut the square into pieces so that the remaining area is equal to the area of the given square.

The perimeter of a rectangular garden is 120 m. Then the garden is expanded as shown to become a rectangular garden. Find the area of the new section open?