Answer to Riddle #88: Work Rate Puzzles

88. Adam can mow a field in 1 hour, in the same time Bob can mow 2 fields, if they work together how long does it take them to mow a field?

And a generalised discussion of work-rate problems.

Philip Jones emailed me with the suggestion that I include a work-rate based puzzle and so I did:

diagram showing three fields squished down into oneBefore reading the answer can I interest you in a clue?

The key to solving this kind of puzzle is to realise it's just a velocity calculation, or speed more accurately. Speed is distance over time.

In Bob's case for example he can do two fields in one hour. This is no different to 2 miles in 1 hour or 2mph. Adam can do 1 field in 1 hour.

Combined they can do 3fph (fields per hour,) so using our equations of motion, time equals distance over speed: they can mow 1 field in ⅓ of an hour, or 20 minutes.

Time for some more complicated problems

If one man can mow a wheat field with a scythe in four twelve-hour days and another man takes three such days, how long will it take them to mow the field working together?
I promise not all of these will involve fields. So work out the speeds, add them, and solve for time.

Person 1: 1 field, 4 days, speed = ¼fpd
Person 2: 1 field, 3 days, speed = ⅓fpd

Adding the speeds 1/4 + 1/3 = 3/12 + 4/12 = 7/12

Time = distance over speed = 1 / (7/12) = 12/7 = 1 + 5/7 = 1 day 8 hours 34 minutes 17.14 seconds
If one man can paint a house in two days, another man can do it in 2.5 days at what rate would a third man have to paint houses for them, working together, to paint a house in a day?
Person 1: 1 house, 2 days, speed = ½hpd
Person 2: 1 house, 2.5 days, speed = ⅖hpd
Person 3: 1 house, x days, speed = 1/xhpd

1 = 0.5 + 0.4 + 1/x
x = 10

It takes the third man 10 days to paint a house. Or, since I asked for the rate, he paints houses at a rate of 1 every 10 days








© Nigel Coldwell 2004 -  – The questions on this site may be reproduced without further permission, I do not claim copyright over them. The answers are mine and may not be reproduced without my expressed prior consent. Please inquire using the link at the top of the page. Secure version of this page.
 


PayPal
I always think it's arrogant to add a donate button, but it has been requested. If I help you get a job though, you could buy me a pint! - nigel






This Website Uses Cookies

To increase the functionality of the site. The cookies I apply do not uniquely identify you, by continuing to use this site you agree to let me place a cookie. I also have advert and analytics providers, my advertising provider (Google,) does provide personalised adverts unless you specify otherwise, with them. For more information click here.x
+