7.
This is a math problem in hexadecimal, which is a base-16 numeral system. We normally use a base-10 system, meaning we count from zero to nine, then start over again and add a digit. However, we don't have any shapes designated for numbers greater than nine, so we use letters; namely, we use the letters 'a' through 'f.' The letter 'a' represents 10, 'b' represents 11, and so forth. So you would count 0,1,2...8,9,a,b,c,d,e,f,11,12...18,19,1a... and so on.
FF represents 255, and F8 is equal to 7 less than 255 (in decimal terms). 255-248=7.
Source(s):
http://en.wikipedia.org/wiki/Hexadecimal
http://www.mrcalculator.com/hexdec.html
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