Explanation: To make a number in cell C1 10% smaller than the number in C3, you would use option (4) =C3*.90. In Excel, to decrease a number by a certain percentage, you multiply the number by (1 - percentage), where the percentage is expressed as a decimal. So, to make a number 10% smaller, you multiply it by 0.90 (which is (1 - 0.10)). Therefore, the correct formula would be =C3*.90. This formula will calculate 90% of the value in cell C3, resulting in a number that is 10% smaller than the original value.