If your talking about decimals 2.1 then:
Step 1) Convert 2.1 into a fraction; 21/10
Step 2) Convert 21/10 into a mixed number. 10 can go in 21 2 times, leaving us with 1. 2 is the whole number and 1 is the numerator. 10 will continue to be the deniometor.
2 1/10
I hope I've helped!