Hi rainy here !!! so lets see lets set up your problem like this
X/1280 = 60%/100 so lets cross multiply
100 times X =100X
60 times 1280 = 76800
100X=76800 now lets divide each side by 100
100 divided by 100 = 0
76800 divided by 100 = 768
X=768
So now we have to subtract to get the total amount so 1280 minus 768 = 512
512 is your answer :)
Hope I helped if u have any questions on how I did your problem feel free to message me or comment on my page bye !!!
Love ~Rainy~