If a gas-filled balloon rises 40 feet in the 1st minute, 60 feet in the 2nd minute, and 80 feet in the 3rd minute, how many feet will the balloon rise during the 15th minute?
a. 300 feet
b. 320 feet
c. 340 feet
d. 360 feet
e. 380 feet

The distance traveled by the balloon in every minute makes an arithmetic sequence with a first term equal to 40 ft and common difference equal to 20. The nth term of an arithmetic sequence is calculated by,
                                     an = a1 + (n - 1) x d
Substituting,
                                      an = 40 + (15 - 1) x 20 = 320
Thus, the answer is letter B. 320 feet.