help me solve this problem

i donot understand the c question... can someone help ?


1. Assume that we can solve the engineering problems to burn coal cleanly and capture the carbon
dioxide so that we can consider it as one answer to the growing electrical energy needs of the
world. Determine how long the earth’s coal reserves will last for each of the scenarios below.
Document the sources for the key pieces of information you use for these calculations and explain
your methods.
a. Assume coal maintains its percentage of the total electrical energy output.
b. If nuclear, natural gas, renewables, and liquids do not increase beyond their current
levels, coal will have to be substituted and usage will increase more rapidly. What is the
new growth rate? How long will coal then last?
c. If your source for the earth’s reserves underestimates the true amount by a factor of 2,
how much longer will the supply last in each of the two above cases?