这是多恩布什宏观经济学第9章课后题，论坛上有答案（英文的）建议看一下，具体解法如下，（来自英文答案）

5.a. While an increase in government purchases by DG = 10 will change intended spending by DSp = 10, a decrease in government transfers by DTR = -10 will change intended spending by a smaller amount, that is, by only DSp = c(DTR) = c(-10). The change in intended spending equals DSp = (1 - c)(10) and equilibrium income should therefore increase by

DY = (multiplier)(1 - c)10.

5.b. If c = 0.8 and t = 0.25, then the size of the multiplier is

a = 1/[1 - c(1 - t)] = 1/[1 - (0.8)(1 - 0.25)] = 1/[1 - (0.6)] = 1/(0.4) = 2.5.

The change in equilibrium income is

DY = a(DA_o) = a[DG + c(DTR)] = (2.5)[10 + (0.8)(-10)] = (2.5)2 = 5

5.c. DBS = t(DY) - DTR - DG = (0.25)(5) - (-10) - 10 = 1.25