The Full Form of PEMDAS is Parenthesis, exponents, multiplication, division, addition, subtraction.

The acronym for remembering the order of operations is PEMDAS, which stands for Parentheses, Exponents, Multiplication, Division, Addition and Subtraction. A mnemonic device for remembering this acronym is Please E xcuse My Dear Aunt Sally.  

Example: The opening problem 5 + 6(4) would be worked as follows.

5 + 6(4) = 5 + 24  since multiplication comes before addition

            = 29

 A common mistake associated with this acronym is to forget that PEMDAS does not properly reflect the left-to-right part of the rules in the first paragraph. Multiplication and division have the same priority, and are done left-to-right. Addition and subtraction have the same priority, and are done left-to-right. A more accurate mnemonic might be   PE MD   AS to remind yourself that the underlined pairs are done together, moving left-to-right.

That is, multiplication and division are done in the same step, and they are done left-to-right.  

Example: In 100 ÷ 4 × 5, the multiplication and division must be done left-to-right, which means that in this case the division is actually done before the multiplication.

100 ÷ 4 × 5 = 25 × 5 = 125

In the same way, addition and subtraction are done in the same step, and they are also done left-to-right.  

Example: In 20 – 2 + 8, the addition and subtraction must be done left-to-right, which means in this case the subtraction is actually done before the addition:  

20 – 2 + 8 = 18 + 8 = 26