honeyhani wrote:

IMO, the correct answer is C. lets just simply pluggin some numbers. imagine we have a total number of 100 bulbs of which 20 are defective and the rest are none-def. the ratio of def to non-def will be 20 to 80 which is equal to 1 to 4. however if we calculate the probablity based on given info, the probability of ocurring defect in bulbs will be 20 to 100 wich is equal to 1 to 5. as you see, the out come is different. we need to know the total number of light bulbs in order to calculate the probability.

Well, in the case you proposed, imagine the only information you have is 20/80, or 1/4. Simply calculating 20+80=100 gives you the rate 20/100 or 1/5. And since probability is expressed in ratios or percentages, then 1/5 is already the solution. So you can calculate the total knowing the partial amounts.

In this exercise, with statement 1 you know that the ratio of defective to nondefective is 1/60. That means that defective to total is 1/(1+60), or 1/61. Then statement 1 is sufficient. You don't need to know the actual number of light bulbs, just the rate between defective to nondefective, or defective to total, or nondefective to total.