Evaluate the expression : common log of square root 9/25.

You want log √ (9/25). Recognizing that √9 = 3 and that √25 = 5, we get

log 3/5, which by rules of logs comes out to log 3 - log 5.

To four decimal places:

log 3 - log 5 = 0.4771 - 0.6990, or - 0.2218.