Math prob help plz

[jinzabread]

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its from math destroyer.
$1 is equal to 7 drachmas, 5 drachmas is equal to 3 quilos.
what is to ratio of dollars to quilos?

A. 3/5
B. 7/5
C. 5/7
D. 5/21
E. 21/5

answer says E but I don't understand why its not D.
help please!!

 

[casper22]

Here is how I would set this up

Equation 1: $1 = 7d

Equation 2: 5d = 3q

From equation 2,

Since 5d = 3q, how many q will be in 7d. Let us say this is X.

thus, 5d = 3q

7d = Xq
Solving for X, we get, X = (7d * 3q) / 5d = 21q/5.

Hence E is correct.

I hope this helps.

 

[qkchen]

nvm

 

[Baylor2011]

Simple cross multiplication. You're trying to find out how many quilos are in 1 dollar or 7 drachmas. So set it up as: (7 drachmas/x quilos) = (5 drachmas/3 quilos). Cross multiply and you get 21 = 5x. Divide both sides by 5 and you get X = 21/5.

 

[engr2dent]

Thanks for the comments above, but I still have a question (the same one I believe the OP had).

Ratio of dollars to quilos = dollars / quilos, correct?

If so, why would you not be able to do unit conversion:

(1 dollar / 7 drachmas) x (5 drachmas / 3 quilos) = 5 dollars / 21 quilos = 5/21 (answer D)

Thanks in advance.

 

[herkulease]

this question came up before. I personally think the problem comes from that fact that 1 dollar = 21/5 quilos is results in the same amount per usd as 5 dollars gets you 21 quilos.

It is an issue of semantics on displaying monetary exchange rates and ratio/fractions

monetary exchange rates are often published 1USD = foreign amount.

Its never written as 5 dollars = foreign amount.

I would have gone with 5/21 if it wasn't about exchange rates because how we are taught to read ratios as fractions.

 

[Baylor2011]

Thanks for the comments above, but I still have a question (the same one I believe the OP had).

Ratio of dollars to quilos = dollars / quilos, correct?

If so, why would you not be able to do unit conversion:

(1 dollar / 7 drachmas) x (5 drachmas / 3 quilos) = 5 dollars / 21 quilos = 5/21 (answer D)

Thanks in advance.

You've got the ratio thing messing with your head. There isn't a direct connection between the two...the bridge for the gap is drachmas.

1$ = 7 drachmas
5 drachmas = 3 quilos

So what you have to think is...I need to know how many QUILOS are in 7 drachmas (since 7 drachmas = 1$). So with this mindset, you set up the ratio:

7 DRACHMAS (1$).... 5 Drachmas
----------------- = ---------------
X Quilos ...................3 Quilos


Ignore the .'s

 

[engr2dent]

Thanks! I see why now.

this question came up before. I personally think the problem comes from that fact that 1 dollar = 21/5 quilos is results in the same amount per usd as 5 dollars gets you 21 quilos.

It is an issue of semantics on displaying monetary exchange rates and ratio/fractions

monetary exchange rates are often published 1USD = foreign amount.

Its never written as 5 dollars = foreign amount.

I would have gone with 5/21 if it wasn't about exchange rates because how we are taught to read ratios as fractions.

You've got the ratio thing messing with your head. There isn't a direct connection between the two...the bridge for the gap is drachmas.

1$ = 7 drachmas
5 drachmas = 3 quilos

So what you have to think is...I need to know how many QUILOS are in 7 drachmas (since 7 drachmas = 1$). So with this mindset, you set up the ratio:

7 DRACHMAS (1$).... 5 Drachmas
----------------- = ---------------
X Quilos ...................3 Quilos


Ignore the .'s