There are 15 marbles in a box. The marbles are of three different colors: red, white, and black. There's 7 times more white marbles than red marbles. How many black marbles are in the box?

Answer 1

R + w + b = 15

w = 7r

r + 7r + b = 15

8r + b = 15

b = 15 - 8r

Based on these conditions, the ONLY possibilities are

0 red and 0 white, 15 blue

1 red , 7 white, 7 blue