12. If the first two terms of the sequence are -3 and 9, the next few terms are
-3 + 9 = 6
9 + 6 = 15
6 + 15 = 21
15 + 21 = 36
and so on. Continuing in this way, you'd find the 16th term to be 4359.
13. Let x be the cost of 1 floop.
31 floops cost less than $40, so 31x < 40 or x < 40/31.
32 floops cost more than $41, so 32x > 41 or x > 41/32.
Equivalently,
41/32 < x < 40/31
and
41/32 ≈ 1.28125
40/31 ≈ 1.29032
which means x = $1.29.