The average sixth-grade girl in the public schools of Baltimore, according to a recently completed survey, can jump rope 47 times in 30 seconds. But Colette Yarosh, a 12-year-old from Holabird Elementary, can spin a rope under her twinkling toes 150 times in half a minute. "I don't believe it," said her principal. "I didn't believe it either," explains Colette's physical education teacher. "Then I timed her myself. She jumped 150 times in 30 seconds, rested five minutes and then hit the five-jumps-a-second pace again." An aircraft company engineer calculated her rope spin at 180 mph, and reporters and research-busy educators called upon Colette to reveal her training secrets. "I began skipping rope when I was 4 years old," said the champion of Baltimore. "Does that explain it?"