a and b can finish a job in 3 hours, b can finish 7 hours, how long does it take for a to finish the job
1/3 = 1/a + 1/b
1/3 = 1/a + 1/7
1/3-1/7 = 1/a
7/21 -3/21 = 1/a
4/21 = 1/a
a= 21/4 = 5 1/4 hr
+36915 
