Question 188953
Let w = John's speed walking
    r = John's speed running
    
Since distance = speed * time, then:

2w + (0.5)r = 12

r = 2w

Substituting 2w for r, we get

2w + (0.5)(2w)=12

3w = 12

w=4

r = 2(4)

r = 8

Therefore, John walks at 4mph and runs at 8mph