So, we just need to solve for x, representing the miles Yochanan walked. The way we could solve it is by substitution.
x+y=1.5
y=1.5-x
Now we can plug in that new equation for y the top equation, 5x+60y=35
5x+60(1.5-x)=35
5x+90-60x=35
-55x+90=35
-55x=-55
x=1
Yochanan walked for 1 mile.
Hope I didn't mess up for your sake!