Just guess, a carpenter can hold four tables a day, or two people, that is to say, a carpenter can hold 2x4=8 people a day, so a carpenter can make 10 single chairs a day, because stools can only make multiples of 10, so guess 8 times what is the multiple of 10, so 5x8=40. According to the previous conjecture, the carpenter who makes the chair is A, the stool is B, and the equation is AX (2x4) = BX10; A+b=9，a=5，b=4。 I wish you progress in your study.