Here's another one:
Start by putting your game piece on the 31...
Now, move down 4 places (since it's -4)... Just like you'd do on a board game if you got a penalty and had to go back 4:
We land on the 27...
Yeah, you're still counting down... But, if you practice this a lot, you'll soon be jumping back in chunks.
It won't be long before you start to see the answer up in your head! Why? Because you'll see the yardstick up in your head and you'll just be able to THINK about the numbers!
For practice on this, check out The Number Monster for great yardstick problems.