In order to multiply a fraction and a decimal, you have to either change the fraction to a decimal or change the decimal to a fraction then multiply ... for example
1/2 × .8 =
we can change 1/2 to a decimal by doing the division that is described by the fraction... the fraction bar literally means "divided by", so 1/2 means 1 ÷ 2 = .5
Now you can multiply
.8 × .5 = .40 or .4
The other choice would be to convert .8 into a fraction. Look at the place value... the 8 is in the 10ths place so the value is 8 tenths or 8/10. Now multiply
8/10 × 1/2 = 8/20 = 4/5