so to get the answer, we just add the number of bones in your hands and bones in your skull
1/4 (bones in hands) + 1/10 (bones in skull)
we can only add fractions that have the same denominator (bottom number in a fraction) so we change the numbers to have a common denominator:
5/20 + 2/20 = 7/20
so 7/20 of your bones are in your hands or skull