Explanation
Circle the first 3 significant figures. That would be the 1, 2, and 3 at the front. Since the next digit is 9, we'll round the 123 up to 124. Then we replace everything else with zeros. The zeros aren't significant as long as you do not place a decimal point after the last zero.
1,240,000 has 3 sig figs while 1,240,000. has 7 sig figs
It might help to think of it like saying [tex]1,240,000 = 1.24 \times 10^6[/tex] where it's more clear we have 3 sig figs in 1.24
