#### Answer

a) 61 hours
b) 52 hours

#### Work Step by Step

Note, there are 10,000 pounds in 5 tons.
a) We find how much energy it takes to get the stuff stored to 90 degrees F:
$ Q = (10,000)(.68)(5) = 34,000 \ J $
We now find how much energy it takes to melt it:
$ Q = (10,000)(104) =1,040,000 \ J $
We now find how much energy it takes to go to go to 60 degrees:
$ Q = (10,000)(.46)(30) = 138,000 \ J $
Thus, the total time is:
$\Delta t = \frac{138,000+1,040,000+34,000}{20,000}=61 \ hours$
b) We now find how much of this time was at 90 degrees:
$ \Delta t =\frac{1,040,000}{20,000}=52 \ hours$