Aggregate Expenditure Function
The aggregate expenditure function represents the total spending in an economy at a given level of income. It is the sum of consumption and investment in an economy.

Given Information
- Consumption Function: C = 100 + 0.75Y
- Autonomous Investment: Rs. 300

Formula for Aggregate Expenditure
The aggregate expenditure function can be represented as:
AE = C + I
where,
AE = Aggregate Expenditure
C = Consumption
I = Investment

Substitute Values
Substitute the given consumption function and autonomous investment into the aggregate expenditure function formula:
AE = (100 + 0.75Y) + 300

Simplify
AE = 100 + 0.75Y + 300
AE = 400 + 0.75Y

Explanation
The aggregate expenditure function for the economy is AE = 400 + 0.75Y. This equation shows the total spending in the economy at a given level of income. Consumption and investment are the two main components of aggregate expenditure. Consumption is determined by income, while investment is autonomous.
In this case, consumption is given by the function C = 100 + 0.75Y, where Y represents income. Autonomous investment is a fixed amount of Rs. 300. By adding consumption and investment, we get the aggregate expenditure function AE = 400 + 0.75Y, which shows the total spending in the economy at any given level of income.