A good properly functioning calculator is NOT a reliable source for something like this, but anyone can do this sort of calculation on paper in a minute and that seems like a reliable source.  Calculators approximate, and knowing how and when such approximations appreciably affect the bottom line requires keeping your brain in gear.  Talking students out of gullibly believing their calculators is a substantial challenge.  (Talking them out of using a calculator as an anesthetic device is another challenge, but not the same one.)