Calculate the instantaneous velocity for the indicated value of the time (in s) of an object for which the displacement (in $\mathrm{ft}$ ) is given by the indicated function. Use the method of Example 3 and calculate values of the average velocity for the given values of $t$ and note the apparent limit as the time interval approaches zero.
$s=120 t-16 t^{2} ;$ when $t=0.5 ;$ use values of $t$ of 0.4,0.45,0.49,0.499,0.4999