Question

find the missing number in. 5,15,50,120,235,405,?

Answer 1

This is a geometric sequence. You need to take a look at the difference between the differences of each two consecutive numbers.

The difference between the first two numbers is 10. The difference between the 2nd and 3rd number is 35. Therefore, the difference between the two differences is 35-10 = 25.

Also, the difference between the 3rd and 4th number is 70, and we already mentioned that the difference between the 2nd and 3rd number is 35. Therefore, the difference between these two differences is 70-35=35.

You can see the pattern here is that the difference between the differences increases by 10 each time. Therefore, for the last number, the difference between the differences will be 25+10(4) = 65. The difference between the 5th and 6th numbers is 170, therefore the last number is 405+65+170 = 640.

Answer 2

The missing number is 640..

Hope this will help you..