I have a simple method: suppose you want to make a pizza with a diameter of 1200 cm, divide it by 40=30, that is, the big pizza is 30 times the diameter of 40 cm, and then divide it by 30=40, that is, the diameter of the big pizza is 40 times the diameter of 30 cm.

Besides, buy a pair, two in 80 yuan with a diameter of 40 cm and one in 60 yuan with a diameter of 30 cm. 30 divided by 60=0.5 cm, that is to say, 1 yuan can buy 0.5 cm, and then 40 divided by 80=0.5 cm, then each is 1 yuan 0.5 cm. Therefore, no matter which one you buy, it is equally cost-effective.

(No plagiarism, I've been thinking about it for a long time ~ ~ ~ ~ ~)