To learn how to set up and solve this sort of exercise, please try this lesson.A can complete a piece of work in 21 hours. With the help of A, B can do it in 6 1/4 hrs. less time than he could do it alone. How many hrs. does B need to do it alone? What kind of equation do you need to solve this problem?
If you entered the equation as you've posted above (that is, without grouping symbols for the denominator on the right-hand side), then there clearly is no solution.I used this equation: 1/21 +1/b = 1/b-6.25, tried to have it worked out on Mathway and was told it had no solution.