The fastest way to solve this is to not care about the percentage (concentration) first. Just look at the actual weight or volume that is being described.
18 (snack mix) + x (apricots that you don't know) = (18+x) (the new mixture)
18 + x = (18+x) ---> (duh right?)
Next, let's apply the concentration:
For the snack mix it's 20% apricots.
For the apricots that you gonna add, it's pure apricot therefore 100%.
And the desired concentration for the new mix is 25% apricots.
Multiplying the concentrations to their corresponding magnitudes:
18*(0.2) + x*(1.0) = (18+x)*(0.25) ----------- (you can actually use 20, 100 and 25 if you want, doesn't matter)
3.6 + x = 4.5 + 0.25x
0.75x = 0.9
x = 1.2 ounces of apricots