What is the next number in the series: 5, 10, 20, 35, 55, _?

To find the next number in the series 5, 10, 20, 35, 55, it's important to identify the pattern in the differences between consecutive terms.

First, let's calculate the differences between each pair of consecutive numbers:

• 10 - 5 = 5

• 20 - 10 = 10

• 35 - 20 = 15

• 55 - 35 = 20

Now we have the differences: 5, 10, 15, 20. The pattern for the differences is that they are increasing by 5 each time.

Following this pattern, the next difference after 20 should be 25. Hence, the next number in the series would be calculated by adding this next difference to the last known number (55):

55 + 25 = 80.

Therefore, the correct next number in the series is 80. This logical progression through the differences demonstrates how the series is constructed and provides clear reasoning to arrive at the correct answer.