Step-by-step explanation:
Consider ...
x/a = b/c . . . . . find x
Multiplying by the denominator under x gives ...
x = ab/c . . . . the value of the unknown.
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In the case where the unknown is in the denominator, you can invert the ratios and solve as above:
a/x = c/b . . . . . note that x is in the denominator
x/a = b/c . . . . . equivalent equation, solve as above