In a triangle, a midsegment (connecting the midpoints of two sides) has length 7. What is the length of the side of the triangle that is…

By the triangle midsegment theorem, a segment connecting the midpoints of two sides is parallel to the third side and half as long. The midsegment has length 7, so the third side has length $2 \times 7 = 14$. A common wrong answer is 3.5, which comes from halving the midsegment instead of doubling it. Another is 7, which assumes the parallel side is equal to the midsegment, and a third is 21, from multiplying by 3. The midsegment is always the shorter one, so the parallel side must be longer than 7.