Chapter 2 - Kinematics in One Dimension - Problems - Page 50: 57

Speed of the raft, $v\approx 0.767$ $m/s$

Let, down be the negative direction In case of stone: $s=ut+\frac{1}{2}gt^{2}$ $-75=(0)t+\frac{1}{2}(-9.8)t^{2}$ $t=\sqrt \frac{150}{9.8}$ $t\approx 3.91$ $s$ In time $3.91$ $s$, the raft travels a distance $(7-4)=3$ $m$ with an constant speed (say $v$) Hence, $x=vt$ $v=\frac{x}{t}$ $v=\frac{3}{3.91}$ $m/s$ $v\approx 0.767$ $m/s$