Your simplified answer is indeed 2/3 (or 60%, or 0.6) because the fraction 24/36 can be simplified by dividing both the numerator and denominator by 6, which leaves you with 2/3.
X+y=15
0.3x+0.6y=0.4*15=6
solve the system of equation.
Multiply equation one by 0.3: 0.3x+0.3y=4.5
subtract the new from the second equation: 0.3y=1.5
y=5
x=10