john can do a piece

Question

John can do a piece of work in 15 days. If he is joined by jill who is 50% more efficient, in what time will john and jill together finish the work?

a) 10 days b) 6 days c) 18 days d) data insufficient e) 8 days

My approach to the problem is

1/15+1/7.5=1/t
5+10/75=1/t
t=75/15
t=5
 b 
what am i missing?

lagomez · Answer

I am getting 6 days by doing the following:

1/15 + 1/10 = 5/30 = 6 days

The reason I used 10 is because 7.5x = 15 and x = 2 which is 100% more efficient
10x = 15 x = 50% more efficient

gurpreetsingh · Answer

Jill is 50% more efficient => 1.5 times John ( its not 50% of john, 50% more => (1+50%) john = 150% of john)

If john can do a piece of work in 15 days => Jill can complete the work in 15/1.5 = 10 days

Now 1/15 + 1/10 = 1/t => t = 6 days.

Silvers · Answer

Do it by ratios.

When Jill works the unit work is 1

When Joh joins in the collective unit work is 1 + 1.5 = 5/2

Since total effort is 5/2 the time will be 2/5 of the original time.

2/5*15 = 6 days.

mbafall2011 · Answer

I think you already know what is wrong with your approach. Here is another try:

15 Days john finishes 100%
In the same 15 days JIll will finish 150%

Together in 15 days they will finish 250% of the job.

Therefore how many days will it take to do 100 % which is 6 days.

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