A well of internal diameter 14 m is dug 12 m deep. The earth taken out is spread evenly all around the well to form an embankment 7 m wide. What is the height of the embankment, in metres? (Use π = 22/7.)

Earth dug out = π $x{7}^{2}x12=(22/7)\times 49\times 12=1848$ cubic m. The embankment is a ring stretching from $r=7$ m to $R=7+7=14$ m, of area π$(196-49)=(22/7)\times 147=462$ sq m, so the height = 1848/462 = 4 m. The trap 3 spreads the earth over the whole outer circle of 616 sq m, forgetting that the well itself occupies the middle; 12 uses π $x{7}^2=154$ sq m, squaring the width instead of taking the ring; 16 uses 14 m as the well's radius.