Math question (fractions)

[Luminus]

Okay, so here's the word problem:

Kirk used 1/4 of his money to buy dilithium crystals. Then he used 1/5 of his money on souvenirs from shore leave. How much money does he have left?

The answer is supposed to be 11/20, but I can't figure out why. Can anyone help me?

 

[Asbo Zaprudder]

1 = 20/20
1/4 = 5/20
1/5 = 4/20

1 - 1/4 - 1/5 = 20/20 -5/20 -4/20 = (20 - 5 - 4)/20 = 11/20

 

[John Picard]

1/4 + 1/5 = 20/20 = 1
1 represents the whole amount of the money

Now, build your fractions with a common denominator:

1/4 = 5/20
1/5 = 4/20

Add the fractions together

5/20 + 4/20 = 9/20

Subtract your answer from the whole, which is now a fraction:
20/20 - 9/20 = 11/20

 

[ed629]

1/4 x 5/5 and 1/5 x 4/4 = 5/20 + 4/20 = 9/20 then 20/20 - 9/20 = 11/20

 

[Lindley]

The problem is unfortunately ambiguous, because while it's expecting you to interpret it as referring to "1/5th of his *original* currency", it doesn't say that. It would be just as valid to assume that he spent 3/4*1/5 of this original currency on shore leave souvenirs, in which case the result would be 12/20 (3/5).

 

[John Picard]

The problem is unfortunately ambiguous, because while it's expecting you to interpret it as referring to "1/5th of his *original* currency", it doesn't say that. It would be just as valid to assume that he spent 3/4*1/5 of this original currency on shore leave souvenirs.

I'm going to assume that by "shore leave souvenirs" you're referring to hookers and blow

 

[Jadzia]

Kirk used 1/4 of his money to buy dilithium crystals. Then he used 1/5 of his money on souvenirs from shore leave. How much money does he have left?

The answer is supposed to be 11/20, but I can't figure out why.

Did you buy a ST:TOS puzzle book?

 

[Luminus]

Thanks all! It was so easy. I can't believe I couldn't figure it out. The practice lecture DeVry gave me didn't explain it properly. I couldn't remember the details of the question, only the fractions. So I made up the Star Trek parts.

 

[Luckyflux]

The problem is unfortunately ambiguous, because while it's expecting you to interpret it as referring to "1/5th of his *original* currency", it doesn't say that. It would be just as valid to assume that he spent 3/4*1/5 of this original currency on shore leave souvenirs.

I'm going to assume that by "shore leave souvenirs" you're referring to hookers and blow

Love it!

 

[Jadzia]

To generalise: If you are given 1/a + 1/b, the result is

= sum of denominators / product of denominators

= (a+b) / (a x b)

It's a very common calculation in optics when considering the focal lengths of lenses.

 

[The Boy Who Cried Worf]

Shouldn't the question read "how much OF his money does he have left?" You don't know how much money he has unless you know how much he started with.

 

[Luminus]

Shouldn't the question read "how much OF his money does he have left?" You don't know how much money he has unless you know how much he started with.

I thought the same thing, when I read it on the sample test. I'm pretty sure that's the way it was worded (the way I worded it). But I could be wrong.