Work and Time

Work and time questions, comprise a large proportion of questions tested in the math reasoning section of the IPAT. With the right practice it is easy to perform well in these type of questions.  Let’s start with a simple example.

Q) If John can complete a piece of work in 4 days and if Mary can complete it in 2 days. How many days will it take for them to complete the work together?

Invert It!

The question is a good example to introduce the concept of “invert it” which is an invaluable technique to solve work and time questions.  Let’s highlight it with the previous example:

The proportion of work John can do in 1 day = 1/4 which is the inverse of the total number of days to complete the piece of work. Think about this for a second. If it takes him 4 days to complete the entire work, in 1 day he will be done with only one fourth (1/4) of the work.

Similarly, the proportion of work Mary can do in 1 day = 1/2.

Total amount of work they can do together in 1 day = proportion John completes + proportion Mary completes = 1/4 +1/2 = 3/4. Therefore if in 1 day they can together complete three fourth of the work, how many days will it take for them to complete the entire work together? The answer is the inverse of the proportion ie 4/3 or 1.33 days. Take a moment to go through this example again if unclear and internalize the concept of inverting.

Let’s do a few more work time questions in the IPAT format

Q) A alone can do a piece of work in 16 days, B alone can do a piece of work in 12 days. When A,B,C worked together the work was completed in 4 days. How long would it take for C to complete the work alone?

A] 9.6 days

B] 10.4 days

C] 8.5 days

D] 11.2 days

E] 8.8 days

Solution

Solution – A] 9.6 days

Let C take c days to complete the work alone.

Therefore proportion of work completed by c in 1 day = 1/c (inverting days here)

Similarly proportion of work completed by  B in 1 day = 1/12

Similarly proportion of work completed by  A in 1 day = 1/16

Together, the proportion of work they can complete in 1 day = 1/c + 1/12 + 1/16

Now we know together they completed the entire work in 4 days. Hence proportion of work they completed in 1 day = 1/4

Equating the above equations => 1/c + 1/12 + 1/16 = 1/4.

We get c = 9.6 days

Q) A, B and C can complete a piece of work individually in 10, 20 and 30 days. How long (nearest number of days) would it take for A to complete the piece of work if he or she is being helped by B and C every second day?

A] 8.06

B] 6.06

C] 7.06

D] 8.22

E] 9.04

Solution

Solution – C] 7.06

Since B and C work only every second day, their effective work period doubles individually ie 20×2 = 40 days and 30×2 = 60 days. Think about it, if these guys work only every second day, it will take twice as long to complete the piece of work.

Now, having found the effective work periods for B and C, let’s use the familiar concept of inverting it to get to the answer.

Therefore 1/a + 1/b(effective) + 1/c(effective) = total proportion of work completed in 1 day.

Therefore, 1/10 + 1/40 + 1/60 = 0.141666 of the work. Now inverting this again to find total days taken to complete entire work we get 1/0.141666 ~= about 7.06 days

Now let’s throw in a slight twist in the form of individual payments for the work done by multiple parties in the next example.

Q) A, B can complete a piece of work individually in 7 and 10 days. If the contract is for $2000, and together with C, the duo is able to complete the work in 2 days. How much shall C be paid of the contract amount?

A] $880

B] $1000

C] $860

D] $950

E] $1030

Solution

Solution – E] $1030

Let C complete the task alone in c days.

We know 1/7 + 1/10 + 1/c = 1/2 ( Think about why this statement is true – Hint : read previous examples)

Therefore c = 3.88 days.

Next, amount paid to C = Contract amount x (proportion contribution of work done by C in a day/proportion of work done in a day)

Now, in each given day, A contributes 1/7 to the work, B contributes 1/10 to the work and C contributes 1/3.88 of the work.

Therefore amount to be paid to C = $2000 x ((1/3.88)/(1/3.88+1/7+1/10)) = $1030

Want more practice? We offer the most comprehensive IPAT practice set for math reasoning here.

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