"D'uh!" Why didn't I think of that sooner??

There's a "Tips and Tricks" thread somewhere, but I had a "D'uh!" moment last night and wanted to share it.  Then I realized that others must have had similar "D'uh!" moments and reading theirs might save me some of mine.  Thus, this thread!

Here's mine from last night:

I was trying to figure out which races to offer various trades to, and suddenly realized that a simpler way than iterations was to check how many BCs each race would trade me for 100 BCs.  It worked.  D'uh!