Explanation:
you mixed an 8% and a 12% solution together.
so, you get (8%×mass1 + 12%×mass2)/((mass1 + mass2)/2) as resulting % of the combined solution.
since the result is 10%, which is precisely the arithmetic middle
(8 + 12)/2
we see that mass1 = mass2.
in other words : the same quantity (mass) of the 8% solution and the 12% solution was mixed together.
so, 760g = 2×mass
=>
mass = 380g
380g of the 8% solution and 380g of the 12% solution were mixed together.