How do u do this? i need in my hw

To write a mixed number such as 8 and 1/4 as a decimal, you convert the fraction (1/4, in this case), and add the converted fraction to the integer (the 8).

1/4 as a decimal is 0.25. Couple ways you can see this: divide 1 by 4 either by hand or with a calculator.
Or 

Now add the converted decimal 0.25 to 8 to get the answer 8.25