If we have the problem: 3 times 1.2, and if we estimate the product, we get (3 times 1) 3. If you really do multiply it out, then you get 3.6. But what if you didn't know that, and you put 0.36. 0.36 is say off from our estimated product: 3. Therefore, we know that 0.36 is wrong, and 3.6 is correct (or reasonably correct), because 3.6 is close of 3. I hope this helps! :D
