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