The volume of
1000
drops of a liquid is
1
fluid ounce.
what is the volume of
100
drops?

By proportion.  First write down the given information systematically:

[1000] drops of liquid equals (1 fl. oz.)
(100) drops of liquid equals ? oz

For problems that are directly proportional, we cross multiply the numbers in the bottom left and top right (in parentheses), divide by the number across ( in square brackets) from the unknown, namely:

 
100 drops * 1 fl. oz / (1000 drops) = (100/1000)* 1 fl. oz = 0.1 fl. oz.

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