Question: Find out the missing number from the below series: 50, , 150, 200, 250, 300, 
Answer: These types of questions are very good for mental exercise. In various competitive exams, you will find similar questions. The series in this example is very simple to solve but the questions for competitive exams may be little or more tricky and need more practice to solve. Anyways, let us solve this question.
We have two numbers missing in this series, which is at 2nd place and 7th place. Let us observe any two numbers in the series. For example we take 150 and 200. One observation is that difference between the two numbers is 50 (200 − 150).
Now we take other pair, say 200 and 250 − In this case also, difference of two numbers is 50 (250 − 200) and next number is 50 more than its immediate number for e.g.: 250 is 50 more than 200 (which is its previous number). So the series is adding 50 to its previous numbers.
Hence, to calculate 2nd number we simply add 50 to the 1st number (which is also 50). So the 2nd number is 100. And 7th number will be 50 more than 6th number. Which is 300 + 50 = 350
Therefore, 2nd number is 100 and 7th number is 350
