Let x be the amount invested at 7%, leaving 30000-x to be invested at 9%.
The equation is
2320 = 0.07 x + 0.09 (30000 - x)
2320 = -0.02 x + .09(30000)
0.02 x = .09(30000) - 2320
x = ( .09(30000) - 2320) / .02
x = 19000
Answer: $19000 at 7%, $11000 at 9%
Check:
19000(.07)+11000(.09) = 2320, good
Isn't it already in standard form?
The two events are independent, because the fact that you picked a number will not affect the result of the coin, and vice-versa, if you flip a coing and pick a number, the fact that you flipped the coin does not affect the number you will pick.
12.1 is the answer your question