Question 139781
If {{{1/6}}} is in the concrete and {{{2/5}}} is in the water, then {{{1/6+2/5=5/30+12/30=17/30}}} is in either the water or the concrete and {{{1-17/30=13/30}}} is the part above the water.  So if the total beam is x feet long, {{{13x/30=78}}}, which is to say {{{x=(78*30)/13}}}.  You do the arithmetic.