What is the rule of the pattern 1 6 16 31 51?

The gaps are 5, 10, 15, 20, .....

The rule becomes add 5(n-1) to the previous number

So that the second number is found by adding 5 times (2-1) to 1 = 5+1 = 6

and the third number is found by adding 5 times (3-1) to 6 = 10+6 = 16

and the fourth number is found by adding 5 times (4-1) to 16 = 15+15 = 31

and so on....