When a calculation only includes additions and subtractions, we work from left to right.

Example
\begin{array}{rcl}
\blue{13-5} +7 &=& \blue{8} +7 \\
&=& 15
\end{array}

When a calculation only includes multiplications and divisions, we also work from left to right.

Example
\begin{array}{rcl}
\blue{8 : 2} \times 3 &=& \blue{4} \times 3 \\
&=& 12
\end{array}

Multiplications and divisions are always done before additions and subtractions.

Example
\begin{array}{rcl}
7+\blue{5\times 8} &=& 7+\blue{40} \\
&=& 47
\end{array}

Always first calculate what is in between brackets. The calculation between brackets is always done before any other operation.

Example
\begin{array}{rcl}
\blue{(3+5)}\times 8 &=& \blue{8}\times {8} \\
&=& 64
\end{array}