<h3>
Answer is 10:7</h3>
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Explanation
It sounds like you want the decimal number to become a whole number. If so, then one way is to multiply both parts by 10 to move that decimal point over one spot to the right
3*10 = 30
2.1*10 = 21
The ratio 3:2.1 is equivalent to 30:21
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Then notice how 30 and 21 have the greatest common factor 3. Divide both parts by 3 so we can fully reduce this ratio
30/3 = 10
21/3 = 7
The ratio 30:21 simplifies fully to 10:7
The numbers 10 and 7 do not have any factors in common, other than 1, so we cannot reduce 10:7 any further.
