Saplings planted per school in a district drive are grouped as 5-15: 3, 15-25: 7, 25-35: f, 35-45: 5. The mean is known to be a WHOLE number and f lies between 4 and 12. Find f.

Class marks 10, 20, 30, 40 give mean $=\frac{370+30f}{15+f}=30-\frac{80}{15+f}$. A whole-number mean needs $(15+f)$ to divide 80. For $f$ between 4 and 12, $15+f$ runs from 19 to 27, and only 20 divides 80, so $f=5$ and the mean is 26. "1" also makes $15+f=16$ a divisor of 80 but lies outside the stated range; "9" gives 24, and 80/24 is not whole (mean 26.67); "10" gives 25, and 80/25 = 3.2 (mean 26.8).