If 8 boxes weigh 30 pounds that means that 4 boxes are going to weigh lighter so 8/4 is 2 so u do 30/2 so ur answer is 15
To find the cross product, you can set this up as a proportion. Please see the attached paper for one of the proportions you could use.
You would then multiply the numbers that are diagonal from each other.
You would have 21(8) = 12x or
168=12x.