What is the least common denominator of 2/7 and 5/4

28 is the LCD.  You can always get a common denominator by dividing the numbers 7 x 4 = 28.  
To find the LCD I look at the multiples of the larger number(since there will be fewer of them) As I do this I check to see if the other number is a factor of it
7(4 isn't a factor), 14(4 isn't a factor), 21(4 isn't a factor), 28(4 IS a factor)!!!

Ithink you're supposed to multiply 12.50 which is how much he makes per hour and then multiply it by each number then add whats left togrther and there's your answer