Question

Question is below :)

Please explain in detail.

Answer 1

Total number of Box B is x +5
Since 1 white ball is added: total number of white ball in Box B is x+5+1 = x+6
Note that now the number of white ball in Box B is 5+1 = 6 and number of yellow balls remains the same at x

Answer 2

Therefore you have the probability of choosing a white ball as:
(number of white ball) / (total number of balls in Box B) = x / (x+6) [justified]