Instead of giving you the plain answer, which has already been given, I will try to let you see how i did it
First notice that the 0.5 occurs one time, then it doesn't the other one, then it does again etc. 79.5 -- 318 -- 715.5 etc
One can guess that the 0.5 occurs when you divide for a power of two that is one higher the highest power of two in the numerator (e.g. you do 4/8, 16/32 etc)
If you look at the power of two, you find that when appears the 0.5 the highest power of two is 4; so you may guess that you divide by 8 when computing the number.
Then multiply by 8 all the numbers (to see what were the factor of the numerator before you divide by 8) and you find out that the factor 159 is always present.
In fact, you get $636 = 4*159, 2544 = 16*159, 5724=36*159, 10176=64*159, 15900=100*159 $
Nice.
So remember we know $y = 159/8*Something$.
Get rid of the 159 (divide everything by 159) to get
20 --> 4 | 40 --> 16 | 60 --> 36 | 80 --> 64 | 100 --> 100
It's easy to see that the pattern here is (x/10)^2
putting all together we get
$y = 159/800x^2$