So what you want to do here is first find a common denominator.
Let's start with decimal component of the number. In our case it's .1 and .1 can go into the number one ten times, so our common denominator can be 10, so so far we have ?/10 where the ? is our numerator. If we convert the whole number 2 to a fraction with ten as the denominator, we will get 20 since 2 * 10 = 20 and 20/10 = 2.
Now all we have to do is add the .1 to the fraction 20/10, so we would do 20/10 +.1/1 = 20/10 + 1/10 = 21/10.
