This is what I would do, but it doesnt seem to work out for me:
2^21/2^8*(2^8+2^10) == substract 2^8 from 2^21 first; 2^13/(2^8+2^10) ==all you can do know is figure out what 2^8 and 2^10 equals (2^8 = 256 and 2^10 = 1024); 2^13/(256+1024) 2^13/(1280) ==know that 1280 = 2^8 * 5