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.