The best way to think about this question is that probability is always out of a total of 1. For example, someone may have a 1/4 chance of picking a certain marble but they will never have a 2 chance of picking a certain marble (that doesn't make any sense). Therefore, we can imagine that the sum of all three probabilities to pick prizes will be 1. We can then set the sum of the probabilities equal to 1:
x+3x+16x=1
You first combine like terms to get 20x=1. Then you divide both sides by 20 to get x=1/20, which is the value of x. This makes sense because you have a small chance of picking a toy in comparison to picking a pen or pencil.
I hope this helps.
