Multiply the equation:

The solution set is the same, because multiplying both sides of an equation by a non-zero number doesn't change the solution set. In fact, if you rewrite the equation as

Multiplying this by 3 (or whatever number, for all it matters) gives

Now, a product is zero if and only if at least one of the factor is zero. So, either
or 
Since the first is clearly impossible, the second one must be true, which is the original equation.
Let
be the amount of food needed for one man for one day. At the beginning, we have enough food for 400 men for 40 days, i.e.

After 10 days, the 400 men will have consumed

food, impliying that the food remaining is

If 200 more men join the garrison, there are now 600 men. Each of them requires
food each day, and there are
units of food remaining. They will last
days.
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1. y = 4x - 3
2. y = -x + 4
3. y = 1/3x +1
4. y = ----
5. y = 1/7x + 2
6. y = -4x + 2.75
7. y = 3x + 5
8.
9.
10.
11.
12.