$21=3\times 7$  $57=3\times 19$  Because $B$ is the factor both number contains, $B=3$  Thus, $A=7$, $C=19$. 
There are three whole numbers $A$, $B$ and $C$ ($B
eq1$). $A\times B=21$, $B\times C=57$. $A+B+C=$~\uline{~~~~~~~~~~}~. 
- $$20$$ 
- $$23$$ 
- $$24$$ 
- $$29$$ 