Mechanics Gear A drives Gear B. Gear A has $a$ teeth and speed $r_{A}$ in revolutions per minute $(r p m) .$ Gear $B$ has $b$ teeth and speed $r_{B} .$ The quantities are related by the formula $a r_{\mathrm{A}}=b r_{\mathrm{B}}$ Gear $\mathrm{A}$ has 60 teeth and speed 540 $\mathrm{rpm} .$ Gear $\mathrm{B}$ has 45 teeth. Find the speed of Gear $\mathrm{B}$ .