Question 706542
A fifth grader could explain it, but not using systems of linear equations. That requires unnecessary complication.
 
{{{x}}}= number of weekday bleachers seats tickets
{{{y}}}= number of weekend lawn seats tickets
 
Total number of ticket you are willing to purchase is
{{{x+y=7}}} because 
"You want a total of 7 tickets."
 
{{{x}}} weekday bleachers seats tickets will cost ${{{11x}}}
{{{y}}} weekend lawn seats tickets will cost ${{{11y}}}
because both cost $11 per ticket.
 
The total cost for
{{{x}}} weekday bleachers seats tickets plus
{{{y}}} weekend lawn seats tickets is
${{{11x+11y}}}
 
You can spend the total ${{{77}}} to buy
{{{x}}} weekday bleachers seats tickets plus{{{y}}} weekend lawn seats tickets,
spending {{{11x+11y=77}}}.
 
To decide what to buy, you would solve the system
{{{system(x+y=7,11x+11y=77)}}}
 
However, the system is what we call "dependent".
We also call it "undeterminate" because there is no unique solution.
The equations  {{{11x+11y=77}}} and {{{x+y=7}}} are equivalent and dependent,
because they are equivalent equations, with exactly the same solutions.
 
Here is what "equivalent equations" means:
{{{11x+11y=77}}} is what we get from {{{x+y=7}}} by multiplying both sides of the equal sign times {{{11}}},
so all the solutions of {{{x+y=7}}} are solutions of {{{11x+11y=77}}}.
{{{x+y=7}}} is what we get from {{{11x+11y=77}}} by dividing both sides of the equal sign by {{{11}}},
so all the solutions of {{{11x+11y=77}}} are solutions of {{{11x+11y=77}}} {{{x+y=7}}}.
In other words, if you have one equation, are you are given the other one,
you are not getting any new information.
All you know is that you can buy 7 tickets with the $77 you have.
(A fifth grader could tell you that).
You can buy all of your 7 tickets for weekday bleachers seats,
or you can buy all of your 7 tickets for weekend lawn seats,
or you could buy any combination of the two kinds of seats.