Only identical terms can be combined in addition and subtraction. When combining identical terms, the sum or difference of the coefficients is calculated and the letter part remains the same.

Example 1

Simplify the expressions if possible.

a)	[[$ 17 \text{ €} + 4 \text{ €} = 21 \text{ €} $]]​
b)	[[$ 150 \text{ m} -35 \text{ m} = 115 \text{ m} $]]​
c)	[[$ 6x+9x=15x $]]​
d)	[[$ 18 \text{ cm} + 101 \text{ €} $]]​	Cannot be combined, because the terms have different letters.
e)	[[$ 5a-3b $]]​	Cannot be combined, because the terms have different letters.

Example 2

Simplify the expressions.