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Dutchess_III's avatar

Will you double check my math?

Asked by Dutchess_III (43051points) January 16th, 2011

This probably a 2nd grade question, but I’d like some verification on my math:
“If a mile equals 1.609 km and the moon is 2160 mi in diameter, what is its diameter in kilometers?”
I come up with 1342.448726 / 1342.5 km

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12 Answers

Brian1946's avatar

What you did is divide 2160 by 1.609 instead of multiplying it by that same number.
Since it takes more kilometers to equal a distance than it does miles, you should multiply 2160 by 1.609.

ratboy's avatar

1 mi = 1.609 km
2160 * 1 mi = 6129 * 1.609 km

SavoirFaire's avatar

I think @ratboy meant to write:

1 mi = 1.609 km
2160 * 1 mi = 2160 * 1.609 km

Which results in:

2160 mi = 3475.44 km

So like @Brian1946 said, you need to multiply rather than divide.

gailcalled's avatar

I just double-checked @Brian1946 and @SavoirFaire and also got 3475.44.

I find it always helpful to do a simple version, such as 2000×1.7 = 3400. That gets me in the ballpark.

Kilometer is larger than a mile; thus a diameter in kilometers has to be larger than it is in miles.

Shuttle128's avatar

Just a nitpick: A kilometer is not larger than a mile. A mile is 1.609 times larger than a kilometer.

The number of kilometers that equal the same distance in miles is more than the number of miles.

3475 is correct.

ratboy's avatar

@SavoirFaire, thanks. 6129?

Dutchess_III's avatar

@Shuttle128 Ah..yes. That did confuse me!

Dutchess_III's avatar

@gailcalled That’s what was confusing me, because a Km is NOT greater than a mile. It’s less than a mile. Thanks you guys…

SavoirFaire's avatar

@ratboy I can’t even count the number of bizarre typing mistakes I’ve made in my life, especially when using the number pad.

gailcalled's avatar

@Shuttle128: You’re right, of course. Talk about making this problem a confusing one. I remember living with a French family one summer when I was young. If I biked 8 kilometers, it felt like only 5 miles to my legs.

Rarebear's avatar

It’s actually more of a 5th grade question.

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