Find the answer to the table shown below.

height _
lenght 1.0
decimal part .80

what goes in the blank?

Normally if you have the things legnth and height, you and another thing, then normally the other thing would be area and you would have 2 given and 1 to solve. Probably your question is legnth=? Height=1.0 decimal part is confusing, I am guessing that you mean area area=0.8 so use formula 
height times legnth=0.8
1.0 times blank=0.8
divide both sides by 1.0
blank=0.8/1=0.8
blank=0.8
legnth=0.8

height x length=0.8
1.0 times blank=0.8
divide both sides by 1.0
blank=0.8/1=0.8

length=0.8

Dis the solution here