How to solve this mathematical operation as fast as possible? I am trying to overcome a test to get in a website. They ask me to do mathematical operations like this 

"$840 \times 883 + 991 - 109$" 

and many more like this (every time) in few seconds.
I cannot do that even though I am using a calculator and copy/paste.
Is there any trick to solve it? I mean mentally in order to be approved?
Many Thanks.
 A: Note that $840 \cdot 883$ means $840$ copies of $883$; if we add one more copy, then it will be $841 \cdot 883$.
That said, you have here: $840 \cdot 883 + (991 - 109) = 840 \cdot 883 + 882$.
So, by adding just one more to your expression, you would have:
$840 \cdot 883 + 882 + 1 = 840 \cdot 883 + 883 = 841 \cdot 883$.
If we add one, though, we better subtract it back off at the end.
This makes your total $841 \cdot 883 - 1$.
I doubt that more could be expected within "five seconds maximum."
A: I sent them a mail asking about this, the reply was :

Hi Jaideep,
Thanks for trying the test.
There are many ways to tackle this challenge. As the role we are looking to fill is for a Java developer we would expect the candidate to write a program to connect to the page, read the question and submit the answer. It's impossible to do it by hand... the timeout is very small.
Many thanks

I hope you've got your answer now.
A: You can cut the expression from the page, paste it into the google search bar (or maybe a window running python) and copy the answer back. All in five seconds?
