Calculation of work done in a time:
If A does a task in 3 days and B does a task in 4 days. The in one day A does 1/3 work and B does 1/4 work.
Both working together do: ( 1 / 3 ) + ( 1 / 4 ) = ( 7 / 12 ) work. Thus the work requires 12 / 7 days to be completed when both work together.
Calculation of negative work done in a time:
If A builds a wall in 3 days and B builds a wall in 4 days, C breaks a wall in 12 days. The in one day A does 1/3 work and B does 1/4 work.
Total work done in a day = ( 1 / 3 ) + ( 1 / 4 ) – ( 1 / 12 ) = 6/12 = 1/2
Total days to build a wall = 2 days.
Work equivalence method / Calculation to solve time and work problems:
In such problems, concept of man-days is important. Example: If a contractor hires 50 workers to complete a road in 100 days. Then he finds out that in 50 days only 40% of road building is done. So how many more days shall he need to complete the road. How many more men would he need to build his road on time.
Case – I
Here 40% work is done in 50 days so 60% work shall be completed in ‘x’ days. By cross multiplying we get x as 75 days.
Case – II
But if work has to be completed on time then find the man-days. So 40% of the work took 50 men * 50 days = 2500 man-days. Then to complete 60% work lets assume ‘X’ man-days. So we get ‘X’ as 3750 man-days. But days are fixed as 50 so 3750 / 50 = 75 men are needed to finish job on time.
Equating men, women and work:
Suppose 8 men can do a job in 12 days and 20 women can do a job in 10 days. In how many days can 12 men and 15 women do the job.
For solving this again we need concept of man-days:
Work needs = 12 * 8 = 96 man-day or 20*10 = 200 woman-days.
But since work is same quantity we equate both:
96 man-days = 200 woman-days
1 man-day = 2.083 woman-day
Therefore in 1 day, 12 men = 12 * 2.083 = 25 women.
Hence 25+15 women = 40 women work on a job that takes 200 woman-days to complete. So to complete task we need 200 woman-days / 40 woman = 5 days.
Calculating time to pass a pole or man who is stationary:
Time taken by a train of length X metres to pass a pole or a standing man or a signal post is equal to the time taken by the train to cover X metres.
Calculating time to pass a object that has a width:
Calculating time to pass a train moving in same direction:
Suppose two trains or two bodies are moving in the same direction at A m / s and B m/s, where A > B, then their relatives speed = (A – B) m / s.
If two trains of length a metres and b metres are moving in the same direction at u m / s and v m / s, then the time taken by the faster train to cross the slower train = (a+b)/(u-v) sec.
Calculating time to pass a train moving in opposite direction:
Suppose two trains or two bodies are moving in opposite directions at A m / s and B m/s, then their relative speed is = (A + B) m/s.
If two trains of length a metres and b metres are moving in opposite directions at u m / s and v m/s, then time taken by the trains to cross each other = (a + b)/(u+v) sec.
Calculating time to reach respective destinations after crossing each other
If two trains (or bodies) start at the same time from points A and B towards each other and after crossing they take a and b sec in reaching B and A respectively, then
(A’s speed ) : (B’s speed) = (b^1/2: a^1/2)
Practice Exercise: Time, Speed, Distance