The majority of us can't figure out any math problem without the use of electronics, but the Japanese have their own way of teaching multiplication. And it doesn't involve cheating by using that calculator on your phone.
You're going to need a paper and pen for this, so if those two things haven't gone extinct in your household, pull them out.
Write out the problem and begin drawing as many individual lines representative by each number in the problem. Whatever number you're multiplying by should go horizontally. So 21 x 13 would have you drawing two vertical lines as well as one line vertically. And one line horizontally, and so on. Think of it as the worst planned game of Tic-Tac-Toe ever.
Once you're done, just count how many times the lines intersect and you have your answer. So put your phone away. You can do math all by yourself now.
Don't think this will help for this problem: The Internet Is Going Nuts Over This Math Problem From Singapore