## Key Concepts

- Addition and subtraction of two-digit numbers as an equation.
- Solve problems related to addition and subtraction using equations with unknowns in any position.

## Addition of 2-digit numbers using equation

Every problem is converted in the form of an equation and the unknown value in the equation is represented as “?”.

Example:

13 + ? = 54

? + 32 = 75

21 + 33 = ?

### Let’s solve a problem with an equation

- James saw 17 ducks swimming in a lake. Later, he saw that there were total of 39 ducks in a lake. How many ducks have joined after some time?

**Answer: **

**Solution by adding: **

- Solve the equation: 15 + ? = 57

**Answer: **

### Subtraction of 2-digit numbers using equation

Every problem is converted in the form of an equation and the unknown value in the equation is represented as “?”.

Example:

63 – ? = 41

? – 37 = 54

87 – 42 = ?

#### Let’s solve a problem with an equation

- Jun collects 13 marbles and Mara collects 41 marbles. How many more marbles does Mara collect than Jun?

**Answer: **

**Solution by subtracting: **

**Q: Solve the equation: 45 – 29 =? **

**Answer: **

## Exercise

- Mark has 31 books on his bookshelf. He gives some to his friends and now has 17 books left. How many books did he give away?

(Write an equation with ‘?’ for the unknown to model the problem. Then solve and find the answer.)

- Ellie has $34. She earns $59 more by working. How much does she have now?

(Write an equation with a ‘?’ for the unknown to model the problem. Then solve and find the answer.)

- Harry has some chocolates with him. He gives away 16 chocolates and now he has 18 chocolates with him. How many chocolates did he have at first?

(Write an equation with a ‘?’ for the unknown to model the problem. Then solve and find the answer.)

- A fruit seller sells 24 pineapples and 17 bananas. How many fruits did he sell in all?

(Write an equation with a ‘?’ for the unknown to model the problem. Then solve and find the answer.)

- Erika has some red cubes and 11 blue cubes. She has 28 red and blue cubes in all. How many red cubes does she have?

(Write an equation with a ‘?’ for the unknown to model the problem. Then solve and find the answer.)

### Concept Map

### What we have learned

- Addition and subtraction of two-digit numbers as an equation.
- Solve problems related to addition and subtraction using equations with unknowns in any position.