Due to the presence of air resistance, if a body dropped from a height of 20 m reaches the ground with a speed of 18ms$^{-1}$, then the time taken by the body to reach the ground is nearly
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When a problem involves non-uniform acceleration (like air resistance) but doesn't provide the force law, check if a simple kinematic equation can be used to find an approximate answer. The equation $s = \left(\frac{u+v}{2}\right)t$ is valid for constant acceleration, but it can also be interpreted as displacement = average velocity $\times$ time, which can be a reasonable approximation.