Laptop sold by store I in October = 210. This quantity is 71.42% of the total number of laptops in the shop.

Recognize that $71.42\%$ is exactly equivalent to the fraction $\frac{5}{7}$. Therefore, if $5 \text{ units} = 210$, then $1 \text{ unit} = 42$, and the total capacity is $7 \text{ units} = 7 \times 42 = 294$.

Many students try to calculate $71.42\%$ of $210$ instead of setting $210$ as the $71.42\%$ part of the total.

Always memorize common fractional equivalents of percentages like $1/7 \approx 14.28\%$ and $5/7 \approx 71.42\%$ to save significant time in competitive exams.