Читать книгу The Pilot Who Wore a Dress: And Other Dastardly Lateral Thinking Mysteries - Tom Cutler, Tom Cutler - Страница 22

The mystery

Jenny Brown and Margaret Green are lifelong friends. They grew up together, they went to the same school together, and they graduated from teacher-training college together. Both of them applied for a teaching post at their local village primary and they were appointed at the same time, in the same September of the same year.

Jenny and Margaret now teach in that school, in adjacent classrooms. The school is a charming Victorian building with a steep tiled roof, and roses round the door. It smells, as many schools do, of shepherd’s pie and pine disinfectant. It has about 120 children each year and at the end of their four years most of them feed into the large secondary school in the town.

Jenny and Margaret’s school is a happy place, with a good head, good staff, generous playgrounds, a large sports field and plenty of trees. Not so long ago a local supermarket offered a great deal of money to buy the bottom end of the cricket pitch, but the headmistress, Miss Jean Piaget, had other ideas. The parents carried her in triumph on the day the supermarket abandoned its scheme (they carried her metaphorically, that is).

One day the two young teachers were sipping tea in the staffroom and discussing mathematics. They decided to teach their pupils that maths is not just for passing exams but is a useful and fascinating subject in the real world. They devised a lesson plan in which the children in their classes would measure the length of every child’s arms and deal with the numbers in different ways, to arrive at the three different sorts of average: the mean (got by adding up all the different lengths of the children’s arms and dividing this figure by the number of children in the class), the median (arrived at by listing in order the different lengths of the children’s arms and finding which arm length falls in the middle of the list) and the mode (found by seeing which arm length occurs most often).

On Monday morning Jenny and Margaret called their respective registers. There were 28 children present in each class, with no absences.

They then explained the task to their classes and allowed them to decide who would be in charge of the tape measure, who would take down all the measurements and who would check the figures before handing in the final calculations. The children got to work, and by lunchtime the numbers were all written down.

In the staffroom Jenny and Margaret compared lists and checked the maths. Miss Tijdelijk, a temporary supply teacher, was passing through with a sandwich and asked Margaret and Jenny what they were doing. They showed her the numbers and to her utter astonishment she discovered that, although everything had been done in exactly the same way in both classrooms, and although all the measurements were correct and all the mathematics properly done, the average (mean) arm length of the children in Jenny’s class was three inches greater than the mean arm length of the children in Margaret’s class.

The problem

The children in both classes are all physically normal, and nobody in either class has extraordinarily short or long arms. The arithmetic is correct and, in fact, accurately reflects the actual arm lengths of the children.

How is it that the children in Jenny’s class appear to have significantly longer arms than the children in Margaret’s class?

Tap here for the solution.