It is not clear why you would wish to add them!
Changing them to fractions is generally the better option because it averts rounding errors.
You change fractions into decimals by dividing their denominators into their numerators as for example 3/4 = 0.75
Yes providing you change the fractions into decimals or change the decimals into fractions
Fractions don't repeat, decimals do. 4/9 = 4 divided by 9 = 0.4444 repeating
All numbers can be changed from fractions to decimals.
change them to decimals
You cannot. They are two different types of decimals.
Yes
To change fractions into decimals divide the numerator by the denominator as for example 3/4 = 0.75
Divide the denominator into the numerator.
No but for fractions
0.1250 as a fraction is 1/8
Not sure what you mean by a mixed decimal, but 5/7 is a repeating decimal: 0.71485 The last five digit repeat. A mixed decimal is when you have both a fraction and a decimal in your answer. Like, 5/7 is equal to ( 0.71 3/7). Fractions equivalent to repeating decimals may be written as mixed decimals.