Question

A jar contains only black marbles and white marbles. If two thirds of the marbles are black, how many white marbles are in the jar?

Answer 1

1) probability of first blacck marble equal 2/3. if we multiply this probability on probability second black marble, we receive common probability 5/12

Let's x be second probability

2/3∗x=5/12

x=5/8 and this probability after we remove one marble, so initially we have 9 marbles in total and 1/3 of them white: 3 white marbles

Sufficient

2) let's x be total marbles

1/3x−1=1/4(x−1)

x=9 marbles in total and 1/3of them white: 3 white marbles

Sufficient.