最后一次求助该题

"in the signaling model, assume high school graduates are paid a stream of income whose present value is $200,000. College graduates are paid a stream of income whose present value is X. College education costs higher-productivity workers $50,000 and lower-productivity workers $150,000. What value of X will cause higher-productivity workers to go to college and lower-productivity workers to not go to college?" 答案是275,000 求问怎么算的