I think peter r has made errors. The only sensible meaning to "the reaction goes to 70% completion" is that only 70% of CaCO₃ is decomposed and we are getting 70% the CaO we could have obtained if all the CaCO₃ had been decomposed. I'm not sure what "70% completion by mass" means – it says nothing like that in the question.
CaCO₃(s) → CaO(s) + CO₂(g)
(a) The mole fraction of solid products shoveled out are 0.7 CaO and 0.3 CaCO₃
(b) The RATIO in pounds is same as the ratio in grams - and only an idiot does chemistry in pounds. So,
0.7 moles of CO₂ are produced for every mole of CaCO₃ placed in the kiln.
Molar mass CaCO₃: 100.08 g
⇒ (0.7 mol)(44 g/mol) = 30.8 g CO₂ produced per 100.08 g CaCO₃
or 0.308 pounds CO₂ are produced per pound of CaCO₃ we start with.