A rectangular piece of land is divided into four smaller rectangles which the areas  are recorded as shown. Find the area of the last rectangle ?

At a kindergarten, each teacher looks after seven children. Later, four children moved to another kindergarder, also there are 2 more new teachers. Now, each teach looks after 5 children. At first, how many children are there in the kindergarten?  Knowing that the number of teachers is under 30?

At first the number of ducks in the pond is 5 times the number of ducks on the ground. After three ducks moving from the ground into the pond , the number of ducks in the pond is eight times the number of ducks on the ground. How many ducks are there altogether? Knowing this number is less than100?

Find the sum of different 3 – digit numbers which can be formed using by digits: 0; 2; 3; 7; 5 and divisible by 15 ?

There are 4 teams in the competition, each team has 5 friends. On the opening day, they shake hands together; each friend just shakes hands a times. If each of them only shakes hands with other teammates and did not shake hands with their own teammates, how many handshakes in total?

The sum of a book’s pages and the number of digits of the number on each page is 428. How many pages does the book have? 

Class 5A has 48 students. During a meeting, boys talk to girls. Tuan talks to 9 girls, Dung talks to 10 girls, Hung talks to 11 girls and Long talks to all the girls in the class. How many boys are there in class 5A?

My younger brother’s current age is 4 times greater than the age of him when the age of my older brother is equal to my younger brother’s current age.The sum of two brothers’ age is 51. How old are they now?

This year An’s age is equal to $\frac{1}{4}$ the age of her brother. 4 years later, An’s age is equal to $\frac{1}{3}$ the age of her brother. How many years ago was An’s age equal to $\frac{1}{9}$ the age of her brother?

Triangle ABC is given. Point N lies on side AB so that AN = 2 x NB. Point M lies on side BC so that BM = MC. Two lines AM and CN intersect at point O. Find the area of the quadrilateral BNOM, knowing the area of the triangle OAN is 8 cm².