S = V(Tb) or V = S/Tb --- call this Equation 1
S = 340(Ts) --- call this Equation 2
where
S = length of the bowling alley
V = speed of the bowling ball
Tb = time for bowling ball to hit the pins after being thrown
Ts = time for sound to travel back to the bowler after pins are hit
Tb + Ts = 2.5
Ts = 2.5 - Tb and substituting this into Equation 2,
S = 340(2.5 - Tb) or solving for "Tb"
Tb = 2.5 - S/340 = 2.5 - 16.5/340
Tb = 2.45 sec.
Substitute this calculated value of "Tb" into Equation 1,
V = 16.5/2.45
V = 6.73 m/sec.
hope it helps