$A$ ball is projected vertically up from the ground. Boy $A$,standing at the window of the first floor of a nearby building,observes that the time interval between the ball crossing him while going up and the ball crossing him while going down is $2 \ s$. Another boy $B$,standing on the second floor,notices the time interval between the ball passing him twice,during up and down motion,is $1 \ s$. Calculate the difference between the vertical positions of boy $B$ and boy $A$. (Assume $g = 10 \ m \ s^{-2}$) (in $m$)

• A
  $8.45$
• B
  $3.75$
• C
  $4.25$
• D
  $2.50$