Let's pretend we start driving at Abacus. 
We drive 50 miles east to Barometer.

So we are now 50 miles from Abacus.

Then we drive 30 miles more east to Deka.

Now we are 80 miles from Abacus.

Now we do a U-turn and go 10 miles west back to Calculus. 
This is 10 miles back toward Abacus. 

So now we are only 70 miles from Abacus.

That's the answer.  Actually we had to go through Calculus on
our way to Deka from Barometer, but Calculus was such a small
town we didn't even notice it.

Edwin