Math destroyer help

[emhl123]

"While touring Europe, Alfred finds that $1=7drachmas and 5 drachmas=3 quills. What's the ratio of dollars to quills?"

Here is how i set up the problem:
($1/7D)(5D/3Q)=$5/21Q
Therefore the ratio would be $5:21Q.
The correct answer is 21/5. Could someone explain where I went wrong? I set up the problem similar to how I would set up a stoichiometry problem for GC.

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[Don Kim]

so you get 1 dollar = 21/5 quilos ~ 4.2 quilos. So inherently, dollar is greater than quilos. So 5/21 wouldn't make sense, since that's saying quilos (denominator) is greater than dollar. Just as how 1kg = 1000g and ratio between the two wouldn't be 1/1000. I think it helps to treat these units as variables. So divide both sides by quilos to get 1 dollar / quilos = 21/5.