A number, when decreased by 1%, gives 3960. The number is:

When a quantity is decreased by r%, the resulting value equals the original quantity multiplied by (1 - r/100). This percentage-decrease identity can be reversed: if only the decreased result is known, the original quantity equals that result divided by (1 - r/100).

1. Let the required number be x.

2. Decreasing x by 1% means multiplying x by (1 - 1/100) = 0.99.

3. The condition given in the question translates into the equation 0.99x = 3960.

4. Solving for x: x = 3960 / 0.99 = 4000.

Substituting x = 4000 back confirms the result: 1% of 4000 is 40, and 4000 - 40 = 3960, which matches the value stated in the question.