The 7 x 1 Witch

Dona Clotilde is a very nice lady who lives in a village, in house which number is 71. It's not known why, but she had a reputation for being a witch. Clotilde was eager to watch a football game. One day, she bought a liquid to clean silver. With this, she won a coupon that gave her a number to compete for a ticket to the 2014 World Cup semifinal at Mineirão, the game between Germany and Brazil. The raffle happened and she won the ticket. Clotilde went to the game, Brazil lost 7 x 1, and everyone in the village thought that Brazil had lost that way because of her, poor thing! Her hacking nephew, San Tanaz, taking her aunt's pains, decided to create a computer virus that interfered with mathematical calculations, so that anything involving number 7 in the accounts would become 0.

For example:

3 + 4 = 0

33 + 44 = 0

17 + 11 = 21

8 x 9 = 2

12 x 7 = 0

8 + 9 = 10