the first line is 560
yes.
________
Although this was not a data sufficiency question, it is very helpful to think of the rule in terms of data sufficiency. When you have "n"
linear distinct equations and "n" unknowns, you can solve for any of the unknowns unless:
(1) one or more equations
isn't linear
or
(2) 2 equations
aren't distinct (x+y = 10 is the same as 2x + 2y = 20)
The above are two situations in which you don't have sufficiency even though you have "n" equations for "n" unknowns. The reason is that some of the equations are either not linear or else not distinct.
But there are also exceptions to the rule; situations where you have sufficiency even though direct application of the rule may suggest otherwise:
(1) If you are asked for a
relationship (ie, addition, subtraction, division, multiplication, etc.) between unknowns, typically you will only need "n-1" equations. For instance, if we are asked for the value of "x/y", then the equation "x/y = 10" is sufficient even though we don't know what "x" and "y" are. The reason is that "x/y" is actually just one unknown (you can call it "z"), and so one equation is sufficient. So at the end of the day, you can argue that this isn't really an exception.
(2) If you have a word problem dealing with indivisible objects (as we do in this question), you know that "n" objects must be a positive integer. This information makes sufficiency more likely. For example, if x and y must be positive integers, then the equation "x*y=5" only has two solutions sets: either x is 1 and y is 5 or vice-versa. But if there isn't a restriction on the kinds of numbers that x and y can be--if x and y can be nonintegers--then there are an infinite number of solutions.
(3) Other kind of information that limits the properties of the numbers involved.
