To find out how many students are left-handed, you can calculate 5% of 840.
First, convert the percentage to a decimal by dividing by 100:
\[ 5% = \frac{5}{100} = 0.05 \]
Now, multiply this decimal by the total number of students:
\[ 0.05 \times 840 = 42 \]
Therefore, there are 42 left-handed students at Marieka's summer camp.