Answer:
1/8
Step-by-step explanation:
Imagine the value of all of the t-shirts are equal to 1.
3/5 of all of them are blue, so 3/5 of 1 --> 3/5 * 1 =3/5
5/8 of the blue tshirts are on sale, so 5/8 of 3/5 --> 5/8 * 3/5 = 3/8
1/3 of those tshirts are medium, so 1/3 of 3/8 --> 1/3 * 3/8 = 1/8
So, 1/8 of all of the tshirts are blue, on sale, and medium