b2bt wrote:
What is meant by
the sum of a positive multiple of 2 and a positive multiple of 4First of all does it actually means --- the sum of a positive multiple of 2 and 4?
Multiple of 2 - 2,4,6,8,10,12,14,16,18,20,22,24,26
Multiple of 4 - 4,8,12,16,20,24
What is sum of these
Didn't understand
It means you need to find those numbers which can be written as 2a + 4b where a and b are positive integers.
n = 2(a + 2b)
Note that a + 2b can take all values starting from 3
a = 1, b = 1, a+2b = 3
a = 2, b = 1, a+2b = 4
a = 1, b = 2, a+2b = 5
a= 2, b = 2, a+2b = 6
etc...
Hence the numbers which are "sum of a positive multiple of 2 and a positive multiple of 4" all are even numbers starting from 6 onwards.
n = 6, 8, 10, 12 etc
You can also do this question by figuring out the number of integers which are none of these three: not prime, not odd multiples of 5, not even (6 and above). The reason you will do that is there will be few such numbers since most numbers less than 28 are either even or prime. Also the options are close to 27 so they tell you that there are few such numbers
Start from 1.
1 - not prime, not multiple of 5, not even (6 and above)
4 - not prime, not multiple of 5, not even (6 and above)
Just focus on the odd numbers now and keep ignoring primes: 5/7 - ignore
9 - not prime, not multiple of 5, not even (6 and above)
11/13/15/17/19 - Ignore
21 - not prime, not multiple of 5, not even (6 and above)
23/25 - Ignore
27 - not prime, not multiple of 5, not even (6 and above)
So there are 5 numbers that do not fall in any of these categories out of a total of 27 numbers which are less than 28.
This gives us 27 - 5 = 22 relevant numbers
Answer (D)
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