Letty sent SonofEva this problem on Sunday night, except in her version the question at the end read, "What do you think is his total profit."
The three of us argued about this for two straight days, figuring out different reasoning that would account for all four answers: $0, $10, $20 and $30. ($80's a new one! Wait'll I tell them!
Finally, Hubby and I settled on the same answer -- $20.
My reasoning was the same as c.i.'s: "...he spent a total of $120, and received $140, his net gain is $20..."
Hubby's reasoning was the same as neologist's: "The fact that he bought the same bicycle back is irrelevant. He bought a bicycle for $50 and sold it for $60. Profit $10. He bought a bicycle for $70 and sold it for $80. Profit $10. Gross profit = 2 X $10 = $20"
(Remember, our version of the problem simply asked for the total profit
SonofEva couldn't decide between $20 and $10.
He said it is $20 if you count profit only. (Reasoning: 2 x $10 = $20, same as above)
But it is $10 if you count profit AND loss. (Reasoning same as yitwail's: "... by selling for 60, he makes a $10 profit, but when he buys it again for 70, he loses 10 to break even, and then he makes $10 again by selling for 80, hence he comes out $10 ahead."
So now we're dying to know, yitwail...WHAT IS THE CORRECT ANSWER?!