Question

What is the value of `15-8 -:2`?

Answer 1

`11`

Explanation:

Unless you are using an older understanding of `-:`, the division should be performed before the subtraction.

So:

`15 - 8 -: 2 = 15 - 4 = 11`

In general, multiplication and division have higher priority than addition and subtraction.

If the operations in your expression have equal priority, then work from left to right (except exponents, which work right to left).

To help you remember these conventions, there are a few different mnemonics which people use. One is PEMDAS ...

P - Parentheses.

E - Exponents.

MD - Multiplication and Division.

AS - Addition and Subtraction.

So Parentheses (brackets) are most important. Perform operations in parentheses before other operations.

Then Exponents are next. For example:

`2 xx 3^2 = 2 xx 9 = 18`

Not:

`color(red)(cancel(color(black)(2 xx 3^2 = 6^2 = 36)))`

Note that MD does not mean that Multiplication has priority over Division. They have equal priority. For example:

`6 -: 3 xx 2 = 2 xx 2 = 4`

Not:

`color(red)(cancel(color(black)(6 -: 3 xx 2 = 6 -: 6 = 1)))`

since division and multiplication have equal priority, we work from left to right.

Similarly AS means that Addition and Subtraction have the same priority as one another.