Calculate molecular weight composition

We know that 450mg of sodium are contained in a certain quantity of $Na_2S_2O_3$. How many grams of sulfur are there in the same quantity?

SOLUTION:
The molecular weight of $Na_2S_2O_3$ is:
$2\cdot23+2\cdot32.07+3\cdot16=158.14gr/mol$
The sodium percentage is:
$$\%Na=\frac{2\cdot23gr/mol}{158.14gr/mol}\cdot100=29.1\%$$

So, in 100gr of $Na_2S_2O_3$ there are 29.1gr of sodium; in $X$ grams of $Na_2S_2O_3$ there are 0.45gr of sodium:
$$100gr:29.1gr=X:0.45gr\Rightarrow X=\frac{0.45\cdot100}{29.1}=1.55gr$$

The sulfur percentage is:
$$\%S=\frac{2\cdot32.07gr/mol}{158.14}\cdot100=40.55\%$$

So, in 100gr of $Na_2S_2O_3$ there are 40.55gr of sulfur; in 1.55gr of $Na_2S_2O_3$ there are $X$ grams of sulfur:
$$100gr:40.55gr=1.55gr:X\Rightarrow X=\frac{40.55\cdot1.55}{100}=0.628gr=628mg$$

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PROBLEM 3:
3.5 grams of aluminium are contained in a certain quantity of aluminium sulfate. How many grams of oxygen are contained in the same quantity of the compound?