Answer:
Each new number adds 10 (you could also write the equation 3 + 10n where n is the position of the number in the sequence)
The blanks are 23 and 43
Explanation:
In two numbers it goes up by 20, so in one it will go up by 10.
This means the blanks are 13 + 10 = 23 and 33 + 10 = 43