Citizen E610 Instruction Manual page 88

[How to calculate ratios]
Q : 30/20 = 60/A
A : Adjust 30 of the outer scale until it points to 20 of the inner scale.
You can read the digit 40 from of the scale of the inner scale corresponding to
60 of the outer scale and on all the positions above the scale, the ratio between
"inner and outer" is the ratio between "30 and 20". Therefore, you can determine
the answers of other ratios.
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