Try it

Do the sum. What is your answer? If you got 5000, you're wrong. Try again.

Still stuck?

Try adding up the numbers out loud, slowly. If all else fails, use a calculator.

Why is the addition so "difficult"?

Main reason; everyone is arrogant. You look at that and think it's easy. Your brain adds it up without really caring. This forges a precedent. So, when you find out your wrong, your brain still remembers the first 'wrong' answer. It sticks to it. If you get it wrong first time, it is really difficult to correct yourself. The same happens in courts of law. When a jury member suggests a fine amount, in most circumstances, the final decision will be anchored towards that suggested figure. It's human nature, and this deception takes full advantage of it.