#"10.0005 g"# has 6 significant figures . The zeros are significant because they are between two significant figures, the 1 and the 5.
#"0.003423 mm"# has four significant figures (the leading zeros are not significant)
#"2900+100=3000 ft"# When adding or subtracting, the number of decimal places in the answer has the fewest number of decimal places as the number of decimals in the problem. So, since both numbers were whole numbers, the answer must be a whole number, and in this case, there is one significant figure in 3000 . There are 2 significant figures in 2900 and 1 significant figure in 100.
#8.9xx10^5"L"# has two significant figures. Only the coefficient 8.9 is significant.