There are blue, red and green pencils in the box—20 pencils total. There are 6 times more green pencils than blue pencils. There are fewer red pencils than green pencils. How many pencils do you need to take out of the box in order to get at least one red pencil among them?

15

Step-by-step explanation:

Try 1, 2, 3, or 4 blue pencils. Then green is 6 times as many. Red must be the rest to make up 20 total.

No. of blue        No. of green      No. of red

1                          6                           13

2                        12                           6

3                        18                           -1

You can't have 3 blue pencils because 3 blue + 18 green = 21 pencils, and there are only 20.

If you have 1 blue and 6 green, then there must be 13 red, but red must be less than green, and 13 is not less than 6.

The only possibility is

2 blue, 12 green, 6 red

If you start taking out pencils, when you take out the first 14 they may be all blues or green, so only when you take out the 15th pencil do you know for sure there must be 1 green pencil.

step-by-step explanation:

1. practice, practice & more practice

2. review errors

3. master the key concepts

do not try to memorise the processes.

4. understand your doubts.

5. create a distraction free study environment

6. create a mathematical dictionary

7. apply maths to real world problems.