Math problem in the third grade of primary school: Mei Mei and I weigh 30 kilograms, Mingming weighs 25 kilograms, and Gong Hong weighs 35 kilograms. How many kilograms do Meimei and Gong Hong weigh respectively? Let a, b, c and d in turn.

Then a+b=30 ①

a+c=25 ②

a+d=35 ③

①-②=b-c=5 ④。

①-③=b-d=-5 ⑤

②-③=c-d=- 10 ⑥

Then B = 15, C = 10 and D = 20. A = 15。