Shawn earns $3.50 per hour for the first 40 hours worked in a week anjd $20.25 per week, for hours over 40. if he earns $661.50 in one week, how many hours did he work?

If he wins more than 40(3.50) = $140 a week, that means that he worked more than 40 hours, and that is the case. So the questions is how many extra hours did he work in a week to win $661.50, lets call that amount of hours x and propose an equation that describes what is happening:
40(3.50) + 20.25x = 661.50
that means, that he worked 40 hours of $3.50 plus an unknown quantity of x hours payed at $20.25 and the total of that is $661.50, lets solve the equation:
40(3.50) + 20.25x = 661.50
140 + 20.25x = 661.50
20.25x = 661.50 - 140
x = 521.5/20.25
x = 25.75
therefore he worked 25.75 extra hours, which give us a total of 40 + 25.75 = 65.75 total hours worked that week