Question

Look at this series: 31, 29, 24, 22, 17, …..what number should come next ?
A). 15
B). 14
C). 13
D). 12

Answer 1

Hint: If we carefully look at the series there is subtraction of some numbers after every digit. So, we will check the value which is being subtracted and according to that we will find the next number which should come in the series.

Complete step-by-step solution:
The series given in the question is a simple alternating subtraction series. As after every digit the next digit is reduced by some value. So, first we will check that,
$31 - 29 = 2$
$29 - 24 = 5$
$24 - 22 = 2$
$22 - 17 = 5$
So, in this series first the number is subtracted by 2 and then by 5, then again by 2 and then again by 5…and so on.
The last number is subtracted by 5. So, the next number should be subtracted by 2.
So, we will subtract the last number which is 17 by 2. And we will get the next number.
$17 - 2 = 15$.
The number that should come next is 15.
So, option (1) is the correct answer.

Note: Alternating subtraction series is the series in which the same amount of number or numbers are subtracted from each consecutive digit. There is also an alternating addition series, in that series the numbers are being added in each next term.
For example: if we are provided with some series like 2, 4, 8, 10, 14, …… and so on. And we are asked to find next. Then, in the series there is addition of 2 at first and then 4. (2+2=4, 4+4=8, 8+2=10, 10+4=14) so, the next term will be last term + 2. (14+2=16). 16 will be the next term.