Question 279281
I think that his first equation was wrong
it should have been: {{{A = B - 9}}}
So then you would have:

{{{A = B - 9}}}
{{{B = C - 5}}}
{{{A + B + C = 56}}}

Use substitution and solve for C
{{{A + B + C = 56}}}
{{{(B - 9) + (C - 5)+ C = 56}}}
{{{((C - 5)- 9) + (C - 5)+ C = 56}}}
{{{3C - 19 = 56}}}
{{{3c = 75}}}
{{{highlight(C = 25)}}}

{{{B = C - 5}}}
{{{B = 25 - 5}}}
{{{highlight(B = 20)}}}

{{{A = B - 9}}}
{{{A = 20 - 9}}}
{{{highlight(11 = A)}}}

There are 11 rats in sewer #1