Math? Huh? Do you understand?
The baker did the wrong equation, the problem he did was: 13 x 7 = 28. The sailer did the correct equation 7x4=28, that is right way to do it. You could rather do 7+7+7+7 or 4+4+4+4+4+4+4 either way would work for the baker. So the sailer was correct and the baker was incorrect.