Hello,
Let's
V1 the velocity of the 1st bricklayer,
V2 the velocity of the 2nd bricklayer,
1wall=V1*6==>V1=w/6
1wall=V2*4==>V2=w/4
==>V1+V2=w/6+w/4=10w*24=5w/12
Let's t the time for the 2 bricklayer to build the wall
(V1+V2)t=w
==>5w/12*t=w
==>t=12/5 (h)=2h24min.
