Question 640803
For short, I'm calling runners only those who completed the race.
(I mean no offense to those who could not finish for whatever reason).
{{{1/8}}} of the total number of runners + {{{7/12}}} of the total number of runners + {{{70}}} runners = confusion
You were adding apples and oranges, or fractions of an unknown number plus the known number 70.
Let {{{x}}} be the total number of runners.
One eight of the total number of runners is {{{(1/8)x}}}.
Seven-twelfths of the total number of runners is {{{(7/12)x}}}.
{{{(1/8)x+(7/12)x+70=x}}}
Multiplying both sides of the equation times 24,
{{{24((1/8)x+(7/12)x+70)=24x}}}
{{{24(1/8)x+24(7/12)x+24*70=24x}}}
{{{3x+14x+1680=24x}}}
{{{17x+1680=24x}}}
{{{17x+1680-17x=24x-17x}}}
{{{1680=7x}}}
{{{1680/7=7x/7}}}
{{{240=x}}} or {{{highlight(x=240)}}}.
So, {{{highlight(240)}}} people completed the race.