A/1 * B/C = D

You discover a chemical reaction that produces 2 moles of valuable propane for every 7 moles of carbon supplied.

Essentially, this is an exchange rate of 2 moles of propane for every 7 moles of carbon.

If you have 1.4 moles of carbon, how many moles of propane can you produce using this reaction?

As per the problem 2 moles of propane are produced when 7 moles of carbon are supplied

so if we supply 1 mole of carbon the moles of propane obtained = 2 / 7

if we supply 1.4 moles of carbon the moles of propane obtained = 2 X 1.4 / 7 = 0.4 moles of propane

Hence answer is propane we can produce using this reaction = 0.4 moles from 1.4 moles of carbon