Question 1090431
{{{M}}} is the midpoint of segment {{{AB}}} and 
{{{N}}} is the midpoint of segment {{{MB}}}. 

A_________M____N____B
.|..........|..........|..........|.......4....|<-

if {{{BN = 4}}}, than {{{MN = 4}}} because {{{N}}} is the midpoint of segment {{{MB}}}

so, {{{MB=8}}}

if {{{MB=8}}} than {{{AM=8}}}because {{{M}}} is the midpoint of segment {{{AB}}}

and  {{{AB=AM+MB=8+8=16}}}