Answer: You must score 15 on the sixth test.
Step-by-step explanation: The set of scores in the question have an average that can be calculated as follows;
Summation of scores/observed data
(Observed data equals 5, that is, five different scores)
Average = (15 + 16 + 17 + 14 + 19)/5
Average = 81/5
Average = 16.2
Now let us assume the target score is X. That means we shall now have observed data as six. That is, 15, 16, 17, 14, 19 and X. The addition of all six observations now becomes 15 + 16 + 17 + 14 + 19 + X and that equals 81 + X
The average of the set of six scores is given as 16. So what we now have is
Average = Summation of scores/observed data
16 = (81 + X)/6
By cross multiplication we now have
16 x 6 = 81 + X
96 = 81 + X
Subtract 81 from both sides of the equation
96 - 81 = 81 - 81 + X
15 = X