A lifecycle assessment (LCA) was conducted to compare 31 gallon steel and plastic (HDPE) fuel tanks for cars. This question is only about the...

First, we must use the scrap rate to calculate the amount of material needed to manufacture each type of tank. Then we multiply the amount of material needed by the primary energy of materials and manufacture, which are given to us in MJ/kg.

For steel:

`(21.92 kg)/(1-0.189) = 27.03 kg`

of steel needed to manufacture one tank. Now, multiplying by the primary energy of materials and manufacture (which are summed first for convenience):

`27.03*(33.5+2.7)=978.5 MJ`

For HDPE:

`(14.07kg)/(1-0.017)=14.31 kg`

Then multiplying the energy of manufacture and materials:

`14.31*(80.94+14)=1359 MJ`

So we can see that the energy costs to produce the HDPE plastic tank are actually quite a bit higher than the energy costs to produce the steel tank, despite the fact that the plastic tank weighs about half of the steel tank.