I used to be able to do these

@HandyAndy

Did you already know this kind of problem or logic it out?

@eladar said
@HandyAndy

Did you already know this kind of problem or logic it out?

I doped it out, knowing that I had to change hours to minutes. Is the solution correct?

@HandyAndy

Yep, but changing units was not the key, combing parts of the job completed in one unit of time was the key.

I was taught..

1/5+1/3=1/t where t is total time.

8/15 =1/t so t= 15/8 or 1 and 7/8 hours.

Just used the calculator 52.5 minutes so you had some round off error I think.

@eladar said
@HandyAndy

Yep, but changing units was not the key, combing parts of the job completed in one unit of time was the key.

I was taught..

1/5+1/3=1/t where t is total time.

8/15 =1/t so t= 15/8 or 1 and 7/8 hours.

Just used the calculator 52.5 minutes so you had some round off error I think.

Both 0.333 and 0.555 are imprecise. I'll bet that's where the rounding error comes in.

@HandyAndy

Yeah, I just put your technique into my calculator using parentheses in one long expression. It came out exactly right.

@eladar said

8/15 =1/t so t= 15/8 or 1 and 7/8 hours.

Just used the calculator 52.5 minutes so you had some round off error I think.

You used a calculator to compute 7/8 of an hour! 😲

@wolfgang59 said
You used a calculator to compute 7/8 of an hour! 😲

Yep, if it was 6th then no problem. I am on a phone with a calculator ap.

@handyandy said
Approximately 1 hour and 53 minutes. Keven does 0.333 percent of the job each minute and Larry does 0.555 percent each minute. Together they complete 0.888 per cent per minute. The total job is finished after 112.6 minutes, or 1 hour and 53 minutes (minus a few seconds).

That is wrong and rather clumsy.
You need to find out the speed at which they work, then add the speeds to find their combined speed.

So K does a third of a job per hour.
And L does a fifth of a job per hour.
1/3 + 1/5 = 5/15 + 3/15 = 8/15

To complete one job will take them 15/8 hours or as Eladar said 1 hour and 52.5 minutes.

@wolfgang59 said
That is wrong and rather clumsy.
You need to find out the speed at which they work, then add the speeds to find their combined speed.

So K does a third of a job per hour.
And L does a fifth of a job per hour.
1/3 + 1/5 = 5/15 + 3/15 = 8/15

To complete one job will take them 15/8 hours or as Eladar said 1 hour and 52.5 minutes.

So you did it the way I showed earlier.

@eladar said
So you did it the way I showed earlier.

With an explanation for HandyAndy.
I guess when one is a teacher one can't stop explaining stuff.

@wolfgang59 said
With an explanation for HandyAndy.
I guess when one is a teacher one can't stop explaining stuff.

He used a rate as well. He chose to go with percent of job completed per second but made the error of rounding off intermediate calculations.

@wolfgang59 said
That is wrong and rather clumsy.
You need to find out the speed at which they work, then add the speeds to find their combined speed.

So K does a third of a job per hour.
And L does a fifth of a job per hour.
1/3 + 1/5 = 5/15 + 3/15 = 8/15

To complete one job will take them 15/8 hours or as Eladar said 1 hour and 52.5 minutes.

The clumsiness was replaced by the proper method. Do you have anything to add to that?

@handyandy said
The clumsiness was replaced by the proper method. Do you have anything to add to that?

I just did.