To write the numbers with the specific number of significant figures, we need to remember that zeros to the left don't count as a significant figure and also that we can add zeros to the right to add significant figures to the numbers.
1) 1.5408 to 3 significant figures, we start counting from left to right, so we will maintain the numbers until the second decimal place, the "8". Since the next is "0", the rounding is "8", so the number is 1.54.
2) Here is similar, but since we don't have decimal places, we need to put the zeros in place of the numbers we are rounding. Since we want 2 significant figures, we will maintain only the first two. However, since the third is greater than 5, we need to round up, so the number is 4400.
3) To get this, we will maintain the zeros to the elft, but they don't count, so we start counting from the "1". Since the third number is less than 5, we round down, so the number is 0.019.
4) Now, here we have an example we need to add zeros. The way it is, 0.5 has only 1 significant figure, so we need to add 3 more zeros to the right to get to 4 significant figures. The number is 0.5000.
5) Here, is the same as item 3, but we want 3 significant figures and since the fourth is greater than 5 we round it up. So, the number is 0.066.