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When I looked at the problem. I thought I should use permutations rather than combinations. My solution was not one of the answer choices and hence switched to Combinations.

Could somebody explain , why combinations should have been the choice in lay man terms.

there are 7+8=15 points in all. "combination" of any 3 points taken at a time will give us a triangle so 15c3=455 BUT if all the 3 points we choose lie on the same line then they will be co-linear and hence wont form a triangle. SO we subtract all such combination's i.e (7c3 + 8c3)= 91 therefore 455-91=364

Permutation is used where arrangements matter. Combination is used here since the order of selection does not matter. i.e. with 3 points chosen (1 point from one line and 2 points from the other parallel line), only a unique triangle can be formed using the 3 points.

Sometimes it's not bad to forget about combinations and permutations and respective formulae. Each point out of 7 on the 1st line can be joined with 8*7 pairs on the 2nd, and any out of 8 the 2nd we join with 7*6 pairs on the 1st. Just don't forget to divide the sum by 2, otherwise each triangle will counted twice. (7*8*7+8*7*6)/2=364.

Last edited by nvgroshar on 19 Jul 2011, 06:54, edited 1 time in total.

If one of two parallel lines has 7 points on it and the other has 8, how many triangles can be drawn using the points?

(C) 2008 GMAT Club - 168 196 316 364 455 Would anyone know how to solve for this?

A triangle needs 3 distinct/non-collinear points to form the three vertices.

Line 1 has 7 points and Line 2 has 8. We cannot select all 3 points from the same line to form a triangle.

we have 2 options. Select 2 points from Line1 and 1 from Line2 (OR) Select 1 point from Line1 and 2 from Line2.

in other words, 7C2*8C1 + 7C1*8C2 (we multiply when we have "AND" and add when we have "OR")

= 364

VP - Would you mind walking me through the math on this. I had the same setup, but got a different answer.

Step 1: Number of combinations of selecting 2 points on Line 1, and one point on Line 2: 7C2 = (7!) / (2!)(7 - 2)! = (7!) / (2!)(5!) = (7*6) / (2) = 21. Now multiply that by 8C1 = 8 to get 168. Step 2: Number of combinations of selecting 2 points on Line 2, and one point on Line 1: 8C2 = (8!) / (2!)(8 - 2)! = (8!) / (2!)(6!) = (8*7) / (2) = 91. Now multiply that by 7C1 = 7 to get 91. Step 3: 168 + 91 = 259

I have a quick question on this problem. When I saw, I immediately did this. Assuming we start on the line with 8 points, I took the number 8. I then multiplied that by 7 (the options on the second line). I then multiplied that number by 7 (the number of options left on the first line). I came out with 392. What did I do wrong? How was my thinking incorrect on this?

I have a quick question on this problem. When I saw, I immediately did this. Assuming we start on the line with 8 points, I took the number 8. I then multiplied that by 7 (the options on the second line). I then multiplied that number by 7 (the number of options left on the first line). I came out with 392. What did I do wrong? How was my thinking incorrect on this?

Hello, and welcome to GMAT Club.

Let me help you on this. There are two flaws in your reasoning: 1. When you pick 2 points from the line with 8 points by 8*7 you'll count twice different pairs of two points from this line. Correct way of choosing 2 different points from 8 would be C^2_8=28 which is half of 8*7=56, so to get rid of these duplication you should divide 56 by 2!.

Consider this: how many different pairs of two letters are possible from ABC - (AB), (BC) and (AC), only 3 (C^2_3=3). If we do your way we'll get 3*2=6 twice as many.

2. Another flaw is that you are only considering the triangles which have two vertices on the line with 8 points and 1 vertex on the line with 7 points, but opposite case is also valid: two vertices on the line with 7 points and 1 vertex on the line with 8 points.

Below are two main approaches for this problem. If one of two parallel lines has 7 points on it and the other has 8, how many triangles can be drawn using the points? (A) 168 (B) 196 (C) 316 (D) 364 (E) 455

Approach #1: There are two types of triangles possible: With two vertices on the line with 8 points and the third vertex on the line with 7 points --> C^2_8*C^1_7=28*7=196; With two vertices on the line with 7 points and the third vertex on the line with 8 points --> C^2_7*C^1_8=21*8=168;

Total: 196+168=364.

Answer: D.

Approach #2:

All different 3 points out of total 8+7=15 points will create a triangle EXCEPT those 3 points which are collinear. C^3_{15}-(C^3_8+C^3_7)=455-(56+35)=364 (where C^3_8 and C^3_7 are # of different 3 collinear points possible from the line with 8 points and the line with 7 points, respectively).

I have a quick question on this problem. When I saw, I immediately did this. Assuming we start on the line with 8 points, I took the number 8. I then multiplied that by 7 (the options on the second line). I then multiplied that number by 7 (the number of options left on the first line). I came out with 392. What did I do wrong? How was my thinking incorrect on this?

Hello, and welcome to GMAT Club.

Let me help you on this. There are two flaws in your reasoning: 1. When you pick 2 points from the line with 8 points by 8*7 you'll count twice different pairs of two points from this line. Correct way of choosing 2 different points from 8 would be C^2_8=28 which is half of 8*7=56, so to get rid of these duplication you should divide 56 by 2!.

Consider this: how many different pairs of two letters are possible from ABC - (AB), (BC) and (AC), only 3 (C^2_3=3). If we do your way we'll get 3*2=6 twice as many.

2. Another flaw is that you are only considering the triangles which have two vertices on the line with 8 points and 1 vertex on the line with 7 points, but opposite case is also valid: two vertices on the line with 7 points and 1 vertex on the line with 8 points.

Below are two main approaches for this problem. If one of two parallel lines has 7 points on it and the other has 8, how many triangles can be drawn using the points? (A) 168 (B) 196 (C) 316 (D) 364 (E) 455

Approach #1: There are two types of triangles possible: With two vertices on the line with 8 points and the third vertex on the line with 7 points --> C^2_8*C^1_7=28*7=196; With two vertices on the line with 7 points and the third vertex on the line with 8 points --> C^2_7*C^1_8=21*8=168;

Total: 196+168=364.

Answer: D.

Approach #2:

All different 3 points out of total 8+7=15 points will create a triangle EXCEPT those 3 points which are collinear. C^3_{15}-(C^3_8+C^3_7)=455-(56+35)=364 (where C^3_8 and C^3_7 are # of different 3 collinear points possible from the line with 8 points and the line with 7 points, respectively).

When I looked at the problem. I thought I should use permutations rather than combinations. My solution was not one of the answer choices and hence switched to Combinations.

Could somebody explain , why combinations should have been the choice in lay man terms.

Thanks a lot

I would suggest to forget about combinations, permutations, factorials... Think of the real process and translate the steps into simple arithmetic operations:

For a triangle, we need three different vertices, which are not all three on the same line. Choose one vertex, say A, from the points on the line with the 7 points and the two other vertices, say B and C, from the points on the line with the 8 points. For A we have 7 choices, and for each of these, for B 8 choices, then for C 7 choices. This will translate into 7 * 8 * 7 choices, but we must divide by 2, because choosing first B, then C, will produce the same triangle as choosing first C and then B. So, we can get 7 * 8 * 7/2 distinct triangles.

Repeat the above process for triangles with vertex A on the line with 8 points, and vertices B and C on the line with 7 points. This will give you 8 * 7 * 6 / 2 triangles.

All the triangles chosen above are distinct. Therefore, the total number of triangles is 7 * 4 * 7 + 4 * 7 * 6 = 28(7 + 6) = 28 * 13 <--- this should end in 4, and luckily, we have just one answer which ends in 4. If there would have been more than one answer which ends in 4, we should have carried out the multiplication or do some other estimate. So, the correct answer is 364.

Answer: D
_________________

PhD in Applied Mathematics Love GMAT Quant questions and running.

To create a triangle we need to select 3 points, 2 from 1st line & one from the 2nd line.

Answer will be 8C2*7C1(we are selecting 2 points from line having 8 points & 1 point from line having 7 points) + 7C2*8C1(we are selecting 2 points from line having 7 points & 1 point from line having 8 points)