David had 20 pencils.5 of them are green and 15 are purple. What percentage of pencils were green?

25%

Step-by-step explanation: Since we know that David has 20 pencils in all and that 5 of the pencils he has are green, we have the fraction 5/20 which represents the 5 green pencils out of all of his pencils.

Now, remember the problem is looking for a percent so we need to write 5/20 as a percent in order to answer the question.

To write a fraction as a percent, first remember that a percent is a ratio of a number to 100. So to write 5/20 as a percent, we need to find a fraction equivalent to 5/20 that has a 100 in the denominator. We can do this by setting up a proportion.

Now, we can use cross products to find the missing value.

So we have 5 times 100 which is 500 is equal to 20 times n or 20n.

So we have 500 = 20n.

Now, dividing both sides of the equation by 20, we have 25 = n.

So 5/20 is equal to 25/100 or 25%.

Therefore, 25% of David's pencils are green.

i think it's eleven too, don't woosh me here, but according to the "experts" it's 2.

the proof starts from the peano postulates, which define the natural

numbers n. n is the smallest set satisfying these postulates:

  p1.   1 is in n.

  p2.   if x is in n, then its "successor" x' is in n.

  p3.   there is no x such that x' = 1.

  p4.   if x isn't 1, then there is a y in n such that y' = x.

  p5.   if s is a subset of n, 1 is in s, and the implication

      (x in s => x' in s) holds, then s = n.

then you have to define addition recursively:

  def: let a and b be in n. if b = 1, then define a + b = a'

      (using p1 and p2). if b isn't 1, then let c' = b, with c in n

      (using p4), and define a + b = (a + c)'.

then you have to define 2:

  def:   2 = 1'

2 is in n by p1, p2, and the definition of 2.

theorem:   1 + 1 = 2

proof: use the first part of the definition of + with a = b = 1.

      then 1 + 1 = 1' = 2   q.e.d.

note: there is an alternate formulation of the peano postulates which

replaces 1 with 0 in p1, p3, p4, and p5. then you have to change the

definition of addition to this:

  def: let a and b be in n. if b = 0, then define a + b = a.

      if b isn't 0, then let c' = b, with c in n, and define

      a + b = (a + c)'.

you also have to define 1 = 0', and 2 = 1'. then the proof of the

theorem above is a little different:

proof: use the second part of the definition of + first:

      1 + 1 = (1 + 0)'

      now use the first part of the definition of + on the sum in

      parentheses:   1 + 1 = (1)' = 1' = 2   q.e.d.