Predict the result
For classroom rounding exercises, add five point two one seven, one point three eight, and two point five.
Which input has the fewest decimal places?

Set up the model
The inputs have three, two, and one decimal places.
Two point five ends in tenths. Round the final sum to tenths.

Work the example
Add without intermediate rounding. The exact sum is nine point zero nine seven.
Round the final sum once, to tenths. The rounded result is nine point one.

Check the result
Now subtract seven point two four nine six from thirteen point five eight.
Choose the decimal place for the answer. Take eight seconds to think.
[Eight-second silent pause to work; the answer remains hidden.]
The exact difference is six point three three zero four. Thirteen point five eight ends in hundredths.
Round the final difference to hundredths. The rounded result is six point three three.

Interpret the result
For these classroom exercises, addition and subtraction use the least precise decimal place among the inputs.
Compute first and round the final result once.
Multiplication and division use a different rule: significant figure counts.