Explanation

Let's break this down step by step:

1. Let the man's monthly income be x.
2. He spends Rs. 3500 per month, which means his savings are x - 3500.
3. He saves 12 ½% of his income, which can be written as a decimal: 0.125x.
4. Since his savings are equal to his income minus his expenses, we can set up the equation: 0.125x = x - 3500.
5. Simplify the equation: 0.125x = x - 3500 --> 0.125x - x = -3500 --> -0.875x = -3500.
6. Divide both sides by -0.875: x = 3500 / 0.875 --> x = 4000.

So, the man's monthly income is Rs. 4000.