Look at this series: $40,40,47,47,54,...$ What number should come next?
$A)40$
$B)44$
$C)54$
$D)61$

Hint: First, we will need to know about the concept of finding the missing sequence numbers. Select the first $2$ numbers and then check the difference or general repetition between them. Again, select the second and third numbers and check the difference between them. In this way, we are able to check all the differences of the given numbers. Hence once the difference or general repetition is found it is easy to calculate the missing number.

Complete step-by-step solution:
Since from the given that we have the series as $40,40,47,47,54,...$ and we need to find the missing number which means the sixth number of the given series.
Let us take the first two numbers from the series and then we will calculate its difference and general repetition. Thus, take the first and second number we have $40,40$
Hence if we see the difference is zero and the first and second numbers are exactly same which means $40 = 40$
Similarly, we are now taking the third and fourth terms of the series. Hence, we have the numbers as $47,47$ and if we see the difference is again zero and also the numbers are exactly the same which means $47 = 47$
Hence this series is a repeated number series and if the fifth number is $54$ is given and hence the sixth number also the same $54$ and hence the option $C)54$ is correct.


Note: Also, the first two numbers and second two numbers difference is seven and hence we are able to solve this problem on that method too. In the given missing number series, we can sometimes find missing numbers at the beginning or at the end of the given series. The layout in the missing number series is similar to the wrong number series, so we have to identify the rule.
Hence here in the given problem, the rule is to find the difference between the two numbers successively.
After that, we solved it very easily using the algebraic operations of addition and subtraction.