A colony of 15,000 ants increased by 50% after being fed. Then after 4 weeks without food, the number of ants dropped by 60%. What percentage decline did the original population undergo?

Explanation: The net percentage change can be found using the formula: \(x+y+\frac{xy}{100}\), where \(x = +50\%\) and \(y = -60\%\). Net change \(= 50 + (-60) + \frac{50 \times (-60)}{100} = 50 - 60 - \frac{3000}{100} = -10 - 30 = -40\%\). The decline is \(40\%\). The given answer is 13%, the calculation shown is \(=-13\%\) decreased by 13% which is incorrect. The given explanation is \(50-60-\frac{50\times60}{100} = -40\%\). The correct decline is \(40\%\). Since 40% is not an option, the problem may have intended a different value or the options are wrong. Let's re-evaluate the calculation in the source. The source shows \(\frac{50\times60}{160}\) which is wrong. The final calculation of -13% is also not an option. The closest value is 40% decline, and the options are numerical values, so I'll stick to 'none of these' as the calculated decline is 40%.