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B alone = 1/(1/12 - 1/20) = 1/(5-3/60) = 60/2 = 30 days.
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Time and Work problems involve finding how long it takes to complete a task. Basic concept: If A can do a work in n days, A's one day work = 1/n. If A and B together can do work in n days, combined one day work = 1/n. Key formula: Total Work = Efficiency x Time. Time and Work is a major topic in SSC CGL, IBPS Bank PO, SBI PO, CAT, and Railway exams.
LCM Method: Assume total work = LCM of given days. Calculate each person's work per day = Total work divided by Days. Combined work per day = Sum of individual work rates. Time = Total work divided by Combined rate. Example: A can do work in 10 days, B in 15 days. LCM(10,15) = 30 units. A's daily work = 3 units, B's = 2 units. Together = 5 units/day. Time = 30/5 = 6 days. This avoids fractions and is much faster.
When worker joins midway: Find work done before and after they join. If A works for x days and B joins for y days, Total work = A's work in x days + (A+B)'s work in y days. Example: A alone in 20 days. If A works 6 days then B joins, to find when work completes: A's 6 days work = 6/20 = 3/10. Remaining = 7/10. If A+B together rate = 1/12, days to finish = (7/10)/(1/12) = 8.4 days.
Pipes and Cistern: Inlet pipe fills cistern (positive), Outlet pipe empties cistern (negative). If inlet fills in x hours, rate = 1/x per hour. If outlet empties in y hours, rate = 1/y per hour. Net rate = 1/x - 1/y. Time to fill = xy/(y-x) hours if y > x. Cistern fills if net rate is positive. These problems are standard in IBPS, SBI, and SSC competitive exams.
Work Equivalence: Men x Days = Women x Days (when efficiency ratio is known). If 1 man = 2 women in efficiency, then 10 men = 20 women. Men-Days concept: 10 men x 5 days = 50 man-days. If 8 men do same work, days = 50/8 = 6.25 days. For wages: divide total payment in ratio of work done. If A and B complete work together and A worked fewer days, calculate individual shares accordingly.
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