You round numbers to the nearest 10 based on the unit value. If it's 5 or above (5, 6, 7, 8, 9), you round up, to the next ten. If it's 4 or below (4, 3, 2, 1, 0), you round down to the same ten.
So 13 has a unit value of 3, so you round down to the same ten, which is literally just the number 10.
Therefore, your final answer is 10 :)
