Answer:
the researcher would need 120 participants.
Explanation:
When dealing with many variables it's possible to find the maximum amount of combinations possible by multiplying the number of variables.
So, we have 2, visual and auditory, 2 levels of difficulty, the amount of material varying to low, moderate and high (3), and the numbers of participants to each unique group is 10.
2·2·3·10 = 120
We can easily find the total amount of participants using this simples equation above.