Question 6090
let x = the amount gained
let -2x = the amount lost
because we are retracing what happened to the money we have to work in the opposite direction:  the gain will be subtracted and the loss will be added back in:
the amount he has now (8000)  minus the gain, plus the loss will equal 10000
8000-x+2x=10000
subtract 8000 from both sides
x=2000

so he gained 2000 and lost twice that or 4000.