Goi STN la a, ST2 la b , ST3 la c Vi 6/7 STN bang 9/11 ST2 nen 6/7a=9/11b a=9/11b : 6/7 = 21/22b (1) Vi 9/11 ST2 bang 2/3 ST3 nen 9/11b=2/3c c=9/11b : 2/3=27/22b (2) Theo bai ra ta co a+b+c=210 (3) Thay (1) va (2) vao (3),ta duoc : 21/22b + b + 27/22b=35/11b=210 ra b= 210:35/11=66 a=66.21/22=63 va c= 66.27/22=81