Best way to tell is to convert them all to either decimals, fractions, or percentages. I usually convert to decimals. So, 75% is 0.75. 0.9 is already a decimal. And 4/5 is 0.8.
So 0.75, or 75%, is now clearly the smallest. In order from least to greatest, they go 75%, 4/5, and 0.9.
