4/5 as a decimal would be 0.8. This is because 5 x 20 = 100, so then if you multiply 5 by 20, then you need to do the same for 4. 4 x 20 = 80. the new fraction is 80/100. Then, that simplifies to 8/10, which is equivalent to eight tenths, which is written as 0.8.