Two equal$-$mass children move along the same line towards one another. The first child moves to the right with speed v$1$i$.$ They collide, hold on to one another, and move together to the right at speed v$1$i where $=0\mathrm{.}4.$ Find the second child's speed before the collision $($v$2$i$)$ in terms of v$1$i$.$ Fill in the missing factor below.