Two paper screens $A$ and $B$ are separated by $150 \mathrm{m}$. A bullet pierces $A$ and $B$. The hole in $B$ is $15 \mathrm{cm}$ below the hole in $A$. If the bullet is travelling horizontally at the time of hitting $A$, then the velocity of the bullet at $A$ is $\left(g=10 \mathrm{m} {\mathrm{s}}^{-2}\right)$

A man swims from a point $A$ on one bank of a river of width $100 \mathrm{m}$. When he swims perpendicular to the water current, he reaches the other bank $50 \mathrm{m}$ downstream. The angle to the bank at which he should swim, to reach the directly opposite point $B$ on the other bank is