Think of N as the total sum of the first set of numbers before they
were divided by 9. So you can say that N / 9 = 9. You can simplify
that to N = 81. Let’s say that M is the new number added. So
(N + M) / 10 = 9. You can simplify that to N + M = 90. Plug
in 81 for N in this new equation: 81 + M = 90
M = 9 (D).