Find the number of multiple of 4 between 7 and 201
Answers
Answer:
First, you need to understand that the sum of all multiples of 7 for n sequences is:
7+14+21+…+7n
Which is:
7(1+2+3+…+n)
Does (1+2+3+…+n) sound familiar to you? If you’re not familiar with that, remember that 1+n = 2+n-1 = 3+n-2 etc and count the number of pairs and multiply the number by the sum of one of the pair
Still don’t understand?
Take a look at the example below:
1+2+3+4+5+6+7+8+9
You will realise that 1+9=2+8=3+7 etc. Next, calculate the number of pairs of numbers with equal sums like the one above by taking the last number in the sum series and dividing it by 2. You will find out that 9/2 is 4.5. Round it down and you find out that there are 4 pairs of equal sums. Multiply it by 1+9, which is 10, and multiply it by 4. Then, find the median in the sequence by rounding up 4.5, which is 5. Consequently, add 40 to 5 and you get 45.