(a) Y = C + I + G + X - M
(b) Closed economy multiplier = \(\frac{1}{1-MPC}\) here \(\frac{1}{1-0.5}\) = \(\frac{1}{0.5}\)
Open economy multiplier = 2
Open economy multiplier = \(\frac{1}{1-MPC \,+\,MP Import}\)
Open economy multiplier = 1.25
the multiplier in closed economy is 2 and open economy is 1.25.