Let F = the sum of the 15 numbers that don't include k and m

So, the average of the 15 numbers $= \frac{F}{15}$

The average (arithmetic mean) of 17 numbers is j.
We can write: $\frac{F + k + m}{17} = j$

What is the average of the remaining 15 numbers in terms of j, k and m?

Take: $\frac{F + k + m}{17} = j$

Multiply both sides of the equation by 17 to get: $F + k + m= 17j$

Subtract k and m from both sides: $F= 17j - k - m$

Divide both sides by 15: $\frac{F}{15}= \frac{17j - k - m}{15}$

Answer: D