1.Vì $21A=14B=6C$ nên $\dfrac{21A}{42}=\dfrac{14B}{42}=\dfrac{6C}{42}$
Suy ra $\dfrac{A}2=\dfrac{B}3=\dfrac{C}7=\dfrac{A+B+C}{2+3+7}=\dfrac{180}{12}=15$
Suy ra $A=30, B=45, C=105$
$2.A-B=22 (1)$ suy ra $A=22+B$
$B-C=22 (2)$ suy ra $B=22+C$
Từ $1$ và $2$ suy ra: $A-B+B-C=44$ suy ra $A-C=44$...