其它
There are three whole number $A$, $B$, $C$. $A\times B=21$, $B\times C=60$. $A+B+C=$~\uline{~~~~~~~~~~}~. 
Overseas Competition->Knowledge Point->Number Theory Modules->Prime Factorization
$21=3\times 7$  $60=2\times 2\times 3\times 5$  Because $B$ is the factor both number contains, $B=3$  Thus, $A=7$, $C=20$. 