A bag has 6 red and 5 blue balls. Four balls are to be selected with equal numbers of both colours. How many selections are possible?

Correct answer: 150

The governing concept is combination, because a selection is made and the order in which balls are chosen does not matter. Since four balls must contain equal numbers of the two colours, exactly two red and two blue balls must be selected. The red balls can be chosen in C(6,2) = 6!/(2!4!) = 15 ways, while the blue balls can be chosen in C(5,2) = 5!/(2!3!) = 10 ways. By the multiplication principle, the total number of selections is 15 × 10 = 150. Therefore, option C is correct. Options A, B and D do not result from the required product and do not correctly count the valid two-red, two-blue selections.