I am trying to figure out how to get the standard deviation for two equations.

The first one is quite simple; it's just how to find the SD for a part of an average value. I have the SD for an average amount of element in 4 values

e.g. average = 118 mg/kg and SD = 48 mg/kg

I am multiplying 118 mg/kg by 0.2 kg to get total element amount in 200 g. How do I calculate the SD for the 200g? Is it simple multiplication there as well?

The second part involves calculating a percent element transfer using the average values from above, as well as average amounts of an element in another medium

i.e. percent = (avg total amount in filtrate/avg total amount in ash) * 100

The avg in filtrate is from 2 values and avg in ash is from 4 values; do I use the relative errors calculation with the answers given from the first question? Does it matter that the number of samples is different?

Thanks for any and all help!