x: liters of 15% orange juice
y: liters of 10% orange juice
x+y=5L
y=5L - x
x liters of 15% orange juice plus y liters of 10% orange juice makes 5 liters of 12% orange juice in equation form:
0.15x+0.10y = 0.12(5L)
0.15x+0.10(5L-x)=0.12(5L)
0.15x+0.5-0.10x=0.6
0.05x=0.1L
x=2L
so y = 3L
so you need 2 liters of 15% orange juice and 3 liters of 10% orange juice