Explanation
The salary with regular pay rate is calculated as:
[tex]S_{\text{regular}}=R\times h[/tex]S is the salary, R is the regular pay rate and h is the number of hours worked.
The salary with over time is:
[tex]S_{\text{overtime}}=2\text{Rxh}_{\text{overtime}}[/tex]Because it pays double after 40 hours.
So the first 40 hours worked you'll get
[tex]S_{\text{regular}}=40R[/tex]The amount of overtime hours is 15 (55-40 = 15), so the overtime salary is:
[tex]S_{\text{overtime}}=2R\times15=30R[/tex]The total Salary is the sum of the regular and the overtime salaries:
[tex]S=S_{\text{overtime}}+S_{\text{regular}}[/tex]If the total salary was $840, then the regular pay rate R is:
[tex]\begin{gathered} 840=30R+40R \\ 840=70R \\ R=\frac{840}{70}=12 \end{gathered}[/tex]Answer
Your regular pay rate is $12 / hour