The temperature of equal masses of three different liquids A, B and C are $12ºC, 19ºC and 28ºC$ respectively. The temperature when A and B are mixed is $16º C$ and when A and B are mixed is $23º C$. What would be the temperature in $ºC$ when A and C are mixed (round off to nearest integer)? 

In accordance with principle of calorimetry : 
 When A and  B are mixed 
 $m{c}_A(16-12)=m{c}_B(19-16)$     
 and when B and C are mixed 
$m{c}_B(23-19)=m{c}_C(28-23) \Rightarrow c_C=(4/5)c_B$     
 Now when A and $C$ are mixed if $T$ is the common temperature of mixture:
   $m{c}_A(T-12)=m{c}_C(28-T)$
 Substituting $c_A$ and $c_C$ from above, 
 $(3/4)(T-12)=(4/5)(28-T)$  
 which on solving gives, 
$T=20.25ºC$