Chapter 7 - Trigonometry and Periodic Functions - 7.1 Introduction to Periodic Functions - Exercises and Problems for Section 7.1 - Exercises and Problems - Page 271: 22

$-13.92 ft/min$

At $t= 2.5 \ min $ we have $h=64.8 \ m $ At $t= 0 \ min $ we have $h=30 \ m $ Vertical Distance $=64.8 \times 30=34.8 \ m $ Now, the average vertical speed is equal to $\dfrac{-34.8}{2.5}=-13.92 ft/min$