A teacher records the minutes of reading that each student in a class did in a week, and groups the…

Step 1. The total. $N = 7 + p + 14 + 10 + 3 = 34 + p$.

Step 2. The step deviations. With $a = 60$ and $h = 20$ the class marks $20, 40, 60, 80, 100$ give $u = -2, -1, 0, 1, 2$. So $\sum fu = -14 - p + 0 + 10 + 6 = 2 - p$.

Step 3. The equation. $58 = 60 + 20 \times \dfrac{2 - p}{34 + p}$. Move $60$: $-2 = \dfrac{20(2 - p)}{34 + p}$.

Step 4. Solve. $-2(34 + p) = 20(2 - p)$, so $-68 - 2p = 40 - 20p$. Then $18p = 108$ and $p = 6$.