Tony can skip at twice the speed that Bruce can, for a speed of 1*2=<<1*2=2>>2 miles per hour.
Brandon can skip at one-third the speed that Tony can, for a speed of 2*(1/3) = 2/3 miles per hour.
Colin can skip at six times the speed that Brandon can, for a speed of (2/3*6=4 miles per hour).
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