Hey there!
Let's keep our goal in mind. A mixed number is a whole number and a fraction together. SInce we already have our whole number, 8, we need to make .12 a fraction.
If we look at .12, we see that that 12 is in the hundredths place. When you go to whole numbers, you have tens, hundreds, and so on. When you go to fractions, it goes tenths, hundredths and so on.
That means that since the 12 is in the hundredths, it's 12 hundredths, or 12/100.
That means our mixed number is 8 12/100
Hope this helps