Problem with proportions I have n elements for a total value of 100.
I have m elements of the n elements each one has a value that is 80% of the value of each other n-m element. With m <= n.
How can I calculate the value of one of the m elements ?

Say that each of the other $n-m$ elements has value $v$. Then each of the $m$ elements has value $0.8v$, and the total value of all $n$ elements is
$$100=m(0.8)v+(n-m)v=(n+0.8m-m)v=(n-0.2m)v\;,$$
so
$$v=\frac{100}{n-0.2m}\;.$$
The value of one of the $m$ elements is $80$% of that, or
$$0.8v=\frac{80}{n-0.2m}\;.$$
