This is an arithmetic sequence, meaning each number increases by a common difference. The formula for arithmetic sequences is aₙ=a₁ + (n-1)(d)
aₙ= The number you are looking for.
a₁= The first term in the sequence. In this case, it is 3.
n= What position the number is in in the sequence. (The first number, the second number, the third number, etc).
d= The difference. To find this, subtract two of the numbers right next to each other (right to left):
8-3= 5
13-8= 5
23-18=5
The common difference is 5.
The formula: aₙ= 3 + (n-1)(5)