Most readers probably had to solve at school quite enough quadratic equations of the type x2–x–6=0. Look then at the following problem. It is typical of the work Professor Zoltan P. Dienes is getting 10-year-olds to do.

“ As many tanks go into battle as the number of each tank crew. A direct hit wipes out one tank, and everyone in it. Six more tank c:ew are lost. At battle's end the survivors pick up some abandoned enemy tanks.