If you wanted to find the mean height of students in an elementary school,

which group would provide the most accurate sample?

It is not possible to select a single group from the primary school, since being a single group it could not in any way represent the entire primary school.

Choosing a group would provide the average height for that group, not the entire primary.

Assumptions can be made that the course of half of the primary would theoretically give us a more accurate representation, but the objective is to select a number of students from each course and there would be a representative sample of the entire primary.

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4