In an open economy, the equilibrium output (Y) is determined where the aggregate demand (AD) equals the aggregate supply (Y). The aggregate demand is the sum of consumption (C), investment (I), government spending (G), exports (X), minus imports (IM). The equilibrium condition can be written as Y = C + I + G + (X - IM). By substituting the given equations and solving for Y, we find the equilibrium output.
The fiscal multiplier is calculated as 1/(1 - MPC(1 - t) + MPI), where MPC is the marginal propensity to consume, t is the tax rate, and MPI is the marginal propensity to import. The current account balance is calculated as X - IM, where X is exports and IM is imports. By substituting the given values and solving, we find the multiplier and the current account balance.