If you're multiplying a whole number by a fraction, then multiply the whole number by the numerator, then divide that number by the denominator.
So in the case of 15 x (5/6), multiply the 15 by 5 (75) and then divide by 6 to get 75/6 (as an unsimplified answer).
In the case of 3 x (3/6), you again multiply the whole number by the numerator (3x3=9) and divide by the denominator (9/6) to get 9/6 or 3/2 when simplified.