A mixed number has a whole number part and a fraction part. To convert the mixed number to decimal you need to take the fraction part and convert it to decimal, then place the whole number before the decimal point.
Example: 5⅜. The whole number is 5. The fraction is ⅜, so we need to convert it to decimal. We can divide 8 into 3 to get 0.375 or we can work out ⅜×1000=375 then divide by 1000 to move the decimal point three places to the left: 0.375. We could have worked out ⅜×10000=3750 then divided by 10000, that is, moved the decimal point four places to the left, but we would get 0.3750 which is the same as 0.375.
So the conversion is 5.375 when we replace the whole number.
Not all fractions work out to a terminating decimal so you need to know how many decimal places are required. ½, ¼, ¾, ⅕, ⅖, ⅗, ⅘, ⅛, ⅜, ⅝, ⅞, ⅟₂₅, ²⁄₂₅, etc., are all terminating decimals; whereas ⅓, ⅔, ⅙, ⅚, etc., are not—they are recurring decimals.