A stone thrown vertically upward from a certain height. It covers half of distance covered during the time of flight to last second. The time of flight (in second) of the stone is__________

From figure,    v=v0−gt1 Or     0=v0−gt1∴    t1=v0g             ........(i) and     v2=v02−2gh or     02=v02−2gh∴    h=v022g             .......(ii)  According to the question,h+H−2h+H2=12gt2−12    ......(iv) From Eqs. (i), (ii), (ii) and (iv), we getTime of flight = t1 + t2 =4s