Walk or Run?

Answer: 56 minutes


Using words

walk, run takes 40 minutes

run, run takes 24 minutes

running takes 12 minutes each way

walk, run takes 40 minutes total, 12 come from running on the way back
So the walking must take 28 minutes.

walk, walk takes 28$\times$2 = 56 minutes.


Using algebra

If it takes John $w$ minutes to walk to (or from) school and $r$ minutes to run to (or from) school, then we have $$\begin{align}r+w&=40\\2r&=24\\2w&=?\end{align}$$

Finding $r$ first
$2r=24\Rightarrow r=12.$ Substituting this into $r+w=40$ gives $12+w=40\Rightarrow w=28.$ So $2w=2\times28=56.$ So it takes $56$ minutes to walk both ways.

Doubling the first equation
Doubling both sides of $r+w=40$ gives $2r+2w=80.$ Since $2r=24,$ $24+2w=80\Rightarrow 2w=80-24=56.$ So it takes $56$ minutes to walk both ways.