23.578 rounded to the nearest tenth is 23.6. Write the error interval for that rounded number. To do the rounding first, use round to a place. For significant figures, use significant figures, then come back with the rounded number and its last place.
23.6 has been rounded to the nearest tenth. Write the error interval.
Use the rounded number, not the original. 23.6 to the nearest tenth is allowed. 23.578 to the nearest tenth is not, because the hundredths digit is still there. A filled dot on the number line is included. An open dot is not.