Fractions can be way easier if you understand that to make a fraction to decimal form, all you got to do is divide the two numbers. So for your case. 4/5 would be .8 and 9/10 would be .9.
.8/.9= .8888889
And we already know .8 (or.9 if your round) is 4/5
Here’s a link to learn another way to convert decimals to fractions: