Surprising result of calculating speeds of two cars hitting a tree

Here's the set up: two cars are side by side on the road. One car is going 70mph and the other is going 100mph. The drivers of the cars see a fallen tree in the road and start braking at the same time. The car that had been going 70mph stops right before touching the tree. How fast is the other car going when it hits the tree? The answer surprised me.