Answer
The rock will fall 49.8 meters.
Work Step by Step
As an object falls freely, the acceleration is $9.80~m/s^2$. We can write this as $9.80~\frac{(m/s)}{s}$. That is, each second, the velocity increases by 9.8 m/s.
As the rock fell, the average velocity was $\frac{40.0~m}{1.00~s}$ which is 40.0 m/s. During the next 1.00-second time interval, the average velocity will increase by 9.80 m/s. Therefore, the rock will fall 49.8 m during the next 1.00 s.
