10/39. If you want it lowest terms that's it. If you are subtracting and putting to a fraction it would be 29/39. If you want a percentage I'm gonna settle about 26% without a calculator. You would divide 1/3.9 and multiply by 100. Or 2.9/3.9 times 100 which I settle at 75% without a calculator. In my head I'm getting 25.4.. and 75.6.. then rounding. Anything else you'll have to specify.
Reduced to a fraction in its lowest terms, 3.9 is equal to 3 9/10 or 3 and nine tenths.
39/10
It is: 3.9 = 39/10 as an improper fraction
-39/10
It is 39/10.
39/10 ml
39/10
-39/10
-3.9 = -39/10
39 7/10
39/10
10/39 10/0.25641025641 = 39.000000000039