$81.$ I$|$ A merry$-$go$-$round is a common piece of playground equipment. A $3\mathrm{.}0-$m$-$diameter merry$-$goround with a mass of $250$ kg is spinning at $20$ rpm $.$ John runs tangent to the merry$-$go$-$round at $\backslash (5\mathrm{.}0\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash ),$ in the same direction that it is turning, and jumps onto the outer edge. John's mass is $30$ kg$.$ What is the merry$-$go$-$round's angular velocity, in rpm$,$ after John jumps on$?$