Step-by-step explanation:
78 = 2 * 39 = 2 * 3 * 13.
Since 6 = 2 * 3 and its factors can be found in 78,
Hence the answer is 6. (C)

he then turns around and grabs that money and sticks it for another 9 years,

add both amounts, and that's how much is for the whole 21 years.
M=6/5 and n = sqrt(2), is rational (but not an integer) and n is irrational.
The case with n = sqrt(9)=3, is not a solution. The case with m=4pi neither.
The last case is m = 6/2=3, integer, so neither works.
So, it is the one with m=6/5 and sqrt(2)
3.5 is rounded to the nearest tenth.