15 counters in the whole set. how many are there in 2/3 of the set?

To figure this out, it'd make the most sense to divide 15 by 3. You get the answer, 5. Since you're trying to find 2/3, you should multiply that by 2. Which would be 10. Therefore, 2/3 of a set of 15 would be 10.

Ok.  they are asking for 2/3 of 15.  of in math always means to multiply

2     15     30
--- × --- =
3      1       3

then simplify 30/3 to 10

2/3 of 15 is 10