use midpoint equation
radius = distance between midpoint and one of the endpoints.
midpoint: (3+5)/2, (2+6)/2, (5+7)/2 = (4,4,6)
equation of sphere: (x-4)^2 + (y-4)^2 + (z-6)^2 = r^2
square distance between midpoint and one of the endpoints.
r^2 = 6
Equation: (x-4)^2 + (y-4)^2 + (z-6)^2 = 6
Let, total number of T-shirts are x.
Number of blue T-shirts = (2/3)x = 2x/3 .
Number of blue shirts on sale = (3/5)(2x/3) = 2x/5 .
Number of medium sized blue shirts on sale = (1/5)(2x/5) = 2x/25 .
Fraction of the shop's T-shirts are blue T-shirts that are on sale and are size medium = = 2/25 . ( Number of medium sized blue shirts on sale divided by total number of T-shirts )
Hence, this is the required solution.