Let's do this, Jocelyn!
Let x = the normal price.
If he bought 16 bottles for $120, we know he bought 12 at regular price and 4 at the reduced price. There are 4 groups of 4 in 16, and every fourth bottle is $3.
So, we have 12x + 4(3) = 120
12x + 12 = 120
12x = 108
x = 9
So, the normal price for a bottle is $9. If he paid $3 each for four, then he saved $6 per bottle. 6(4) = 24.
His total savings is $24.
Hope this helps!