White:Yellow = x:(x+50) = 8:13 --> 13x = 8x + 400 --> x = 80.
The total # of balls originally x + x = 80 + 80 = 160.
Bunuel's algebraic solution above is perfectly correct. You can also solve this question using the concept if you don't like algebra or if you think it's faster:
The ratio of white:yellow balls was supposed to be even, i.e. 1:1 or 2:2 or 8:8, before the extra 50 yellow balls arrived. This means that the number of white balls is unchanged in the new ratio so 8:13 implies that the difference between 13 and 8 (aka 5 units) is representing 50 yellow balls. Each unit in this ratio thus represents 10 balls of indiscriminate colour, making the original 8:8 ratio represent 80 white balls and 80 yellow balls: 160 overall. Answer D.
Algebra never fails in these problems but it is time-consuming for some people, so if you can solve a question multiple ways, you're more flexible on test day to solve tricky or new question types.
Hope this helps
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