To find out how many students are left-handed, we need to calculate 5% of 840 students.
First, convert 5% to a decimal: \[ 5% = \frac{5}{100} = 0.05 \]
Now, multiply 840 by 0.05: \[ 840 \times 0.05 = 42 \]
Therefore, the number of left-handed students is \( \boxed{42} \).