Let the weights of the boxes be , with being the heaviest, the second heaviest and so on.

Then we have:

since the boxes weigh 60 pounds all together

......................(1)

since each box is twice as heavy as the next box

...........................(2)

...........................(3)

...........................(4)

since the lightest box is 20 pounds less than the 2 boxes in the middle combined

.....................(5)

so we have 5 equations and four unknowns, do you think we can find the unknowns?