For Ist part of journey, u=0,a=−g=−10ms−2. ‌s=−40m Now using, v2−u2=2as we get, v2−0=2×(−10)(−40) ⇒v2=800 And using v=u+at, we get; v=0+at1, or v2=a2t12 ⇒t12=
v2
a2
=
800
100
=8⇒t1=2√2 Now, for second part of journey, u=0,a=−10m∕s2,s=−80m ⇒v2−u2=2as⇒v2=2×(−10)(−80)=1600 And v=u+at2 gives, v=at2⇒v2=a2t22 Hence, 1600=100×t22 or t2=4s Time to complete last 50% part of the journey =t2−t1=4−2√2≃1.17s