There are 18 beads in a box. Some of them are white and the remaining are black. The probability of drawing a black from it is 1/3. Then
 a. How many black beads are there in the box?  
 b. How many white beads are there in  the  box? 
 c. How many white beads should be added to it, so that the probability of drawing a black bead become 1/4?

a.
probability = no. of black beads / total no. of beads
1/3 = no. of black beads/18
no. of black beads = 6

b.
probability = no. of white beads / total no. of beads
1 - 1/3 = no. of white beads / 18
no. of white beads = 12

c.
6 white beads should be added

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