Answer: 31.260
Step-by-step explanation: To round 31.26 to the nearest thousandth decimal, we need to look at the digit in the thousandths place, which is the third digit after the decimal point.
The digit in the thousandths place is 6. Since 6 is greater than or equal to 5, we round up the digit in the hundredths place, which is 2, by adding 1.
So, rounding 31.26 to the nearest thousandth decimal gives us 31.260.