Roxanna's normal hourly pay is $ 12
Solution:
Lets assume Roxanna's normal hourly pay = "x" 
As overtime pay is 1.5 times of normal pay 
So Roxanna's overtime hourly pay = 

Also given in 4 hours, in overtime Roxanna earned $72.00 
So in 1 hour, in overtime Roxanna earned = 

That means Roxanna's overtime hourly pay = 18 
Also we have calculated Roxanna's overtime hourly pay in terms of "x" that is 1.5x 
=> 1.5x = 18 
=> 

Roxanna’s normal hourly pay = x = $12