A body is projected upwards with a velocity u. If passes through a certain point above the ground after   $t_1$. The time after which the body passes through the same point during the return journey is 

Suppose V be the velocity attained by the body after time then  $V=u-g{t}_1\_\_\left(i\right)$
Let the body reach the same point at time t . Now velocity will be downwords with same    
   magnitude.then $-V=u-g{t}_2\_\_\left(ii\right)$ 
           $\left(i\right)-\left(ii\right)\Rightarrow 2V=g\left(t_2-t_1\right)$
               $\left(or\right)\text{\hspace{0.33em}\hspace{0.33em}\hspace{0.33em} }{t}_2-t_1=\frac{2v}{g}$
                                   $=\frac{2}{g}\left(u-g{t}_1\right)$
                                   $=2\left(\frac{u}{g}-t_1\right)$