Question 1039775
x=.50 antifreeze
x(0.50)+90 (0.10)=(90+x)(0.40).  One wants the amount of pure antifreeze.
.50x+9=36+0.4x
0.10x=27
x=270 gallons
Check.
40% is 3/4 of the way from 10 to 50
270 gallons is 3/4 of 360 gallons, the total.