PW in engineering economics refers to the present value of expenses, and the demand of a commodity (the need to pay) refers to the number of commodities that consumers are willing and able to buy at various possible price levels in a certain period of time. The main determinants: the price of commodities, the income level of consumers, the prices of related commodities, consumers' preferences and consumers' price expectations of commodities. P: price q: quantity d: demand s: supply e: equilibrium or expectation e: elasticity ed: price elasticity of demand es: price elasticity of supply exy: cross price elasticity of demand u: utility TU: total utility MU: marginal utility CS: consumer surplus.

MRS: marginal substitution rate of goods L: labor K: capital TP: total output AP: average output MP: marginal product MRTS: marginal substitution rate of technology C: cost STC: short-term total cost. TFC: never be this TVC: total variable cost TC: total cost AFC: never be this AVC average: average variable cost. AC: average total cost MC: marginal cost LTC: long-term total cost LAC: long-term average cost SAC: short-term average cost.

LMC: long-term marginal cost SMC: short-term marginal cost TR: total income AR: average income MR: marginal income PS: producer surplus MP: marginal product VMP: marginal product value W: labor price MRP: marginal income product MFC: marginal factor cost R: interest rate PEP: price inflation line.

1. Canada needs to export 1/22 computers to import one unit of wheat. In terms of working hours, it takes 3 hours to produce a unit of wheat in China, but it takes 60/22 hours to produce 1/22 computers in China. Therefore, Canadian import of one unit of wheat is equivalent to exchanging 60/22 hours of labor for 3 hours of products produced in China, saving 3-60/22. China needs to export 22 units of wheat to import 1 unit computer, which is equivalent to exchanging 22*4=88 hours of labor for 100 hours of domestic products, saving 100-88 = 12 hours of labor time.

Second, if the world price is 1 computer to exchange 24 wheat sets, it is equivalent to exchanging 60/24 hours of labor for 3 hours of domestic production, saving 3-60/24 = 1/2 hours of labor time. China needs to export 24 units of wheat to import 1 unit computer, which is equivalent to exchanging 24*4=96 hours of labor for 100 hours of domestic products, saving 100-96 = 4 hours of labor time.