Each of a, b and c need a certain unique time to do a certain work. c needs 1 hour less than a to complete the work. working together, they require 30 minutes to complete 50% of the job. the work also gets completed if a and b start working together and a leaves after 1 hour and b works for a further 3 hours. how much work does c do per hour?

step-by-step explanation:

points a and b are equidistant from the desired line, so the line you find by using them will go through point r.

using points other than a and b will give the desired line only if they are equidistant from r. if they are randomly located, the perpendicular line you create will not necessarily go through point r, hence will not be a solution to the problem.