Question

the probability of guessing the correct answer to certain question is x/3. if the probability of not guessing a correct answer is x/5. find the value of x.

Answer 1

The probability of guessing the correct answer to certain question is x/3. if the probability of not guessing a correct answer is x/5. 
therefore the value of x is 15/8

Answer 2

x/3+x/5=1.... 3x+5x=15..... 8x=15...... x=15/8...... {P(E)+p(E')=1}