real gdp

[ahlashi]

governtment purchases rises by 100$ and MPC=.80
ASSUMING IDLE RESOURCES EXIST AT each ependiture round and the multiplier is operative.
Change in real GDP equals ?

ANS ; 500$

 i am not getting the correct answer pls help

[Dr. Strange]

Y = C + G + cY
Y= (C+G)/(1-c)
dY = dG/(1-c)
 c= 0.8, dG=100 so dY =500