let x=amount of 10% and y=amount of 40%. Since we are making 10oz total, y=10-x so we can say:
(.1x+.4(10-x))/10=.25
(.1x+4-.4x)/10=.25
4-.3x=2.5
-.3x=-1.5
x=5, and since y=10-x, y=5
So 5oz of 10% is mixed with 5oz of 40% to make 10oz of 25% solution.
check
(.1(5)+.4(5))/10=.25
(.5+2)/10=.25
2.5/10=.25
.25=.25