23 is already 23.00, which is in decimal form.
As for 8/25, convert the denominator of the fraction to make it 100 first. So what number do we multiply to 25 to give us 100? 4. And as a rule of fractions, you must multiply both the numerator and the denominator by the same number. So:
(8 * 4)/(25 * 4) = 32/100
Now, to convert a fraction to a decimal, first pay attention to how many zeros there are in the denominator. 100 has 2 zeros. So take the numerator, which is 32, and move the decimal point two places to the LEFT. (Note: The decimal point of 32 occurs right after the "2".)
This will give you 0.32
23.00 + 0.32 = 23.32 <-- This is the decimal form of 23 8/25.