42 ml is the Eukobrom concentrate.
A 2% solution is 20g of solute in 1000g of solution. The proportion is like this:
g solute : g solution = x : 100
We are not considering the fact that 1l of solution is circa 1000g of water + 20g of hypo, that is 1020g total. For simplicity's sake we round it to 1000g.
Taking 125ml of a 2% solution is the same as taking roughly 2,5g of that solute.