You measured the mass of three 25-centavo coins. The measurements are : 3.48 g, 3.5 g and 3.499 g, what is the mass of the three coins?
We're asked to find the total mass of the three coins, while obeying rules for significant figures.
The significant figures rule regarding addition is that the answer contains as many decimal places as the quantity with the fewest number of decimal places.
The quantity with the fewest number of decimal places is