There are 840 students in Marieka’s summer camp. If 5% of them are left-handed, how many students are left-handed?

5% of the students are left-handed, which is (5/100)*840 = <<5/100*840=42>>42 students. Answer: \boxed{42}.

To find the number of students who are left-handed, we need to calculate 5% of 840.

Step 1: Convert 5% to decimal form, by dividing it by 100: 5/100 = 0.05.

Step 2: Multiply 0.05 by 840 to find the number of left-handed students: 0.05 * 840 = 42.

Therefore, there are 42 left-handed students in Marieka's summer camp.

To find out how many students are left-handed, you can follow these steps:

Step 1: Calculate 5% of the total number of students.
To find 5% of a number, multiply the number by 0.05 (since 5% is equivalent to 0.05).

In this case, you have 840 students, so you can calculate 5% by doing:
5% = 0.05 * 840

Step 2: Calculate the result of the multiplication.
0.05 * 840 = 42

Therefore, there are 42 left-handed students in Marieka's summer camp.