Alvin and Bob can complete a job in 72 days. Bob and Catherine can complete it in 120 days, Alvin and Catherine can complete it in 90 days. In how many days can Alvin alone do the work ?
1/a + 1/b = 1/72 or 1/b = 1/72-1/a
1/b + 1/c = 1/120
1/a + 1/c = 1/90
RED into second equation 1/72 - 1/a + 1/c = 1/120 or 1/c = 1/120-1/72 + 1/a sub this into the third equation
1/a + 1/120 -1/72+1/a = 1/90
2/a = 1/90 - 1/120 + 1/72
a = 120 days