An "Average" Problem

Doug, Dick, and Dean are triplets in Mr. Finch's first-period Algebra I class, but Friday's exam is to be their last in that school.

(Their father is in the military, which frequently reassigns him, so the boys sometimes change schools even in the middle of the semester.)

On the following Monday, after the triplets have left the school, Mr. Finch announces the class averages.

"Before Doug, Dick, and Dean left, the class average was 74% -- not bad, really. However, removing their scores causes the class average to rise by one percentage point. Okay?"

"Furthermore," he adds, "the ratios of the triplets' scores is 6 : 5 : 3. What are the scores of our three departing friends?"

It didn't take long for all dozen students in the class to arrive at a satisfactory answer.

BONUS: There is a more precise mathematical term than " average" to describe this situation. What is it?