Subtract the two given numbers:
81 - 3 = 78
Divide by the number of missing numbers +1, so divide by 3:
78/3 = 26
This gives you the difference of each number in the sequence, so now subtract 26 from each one:
81 -26 = 55
55 - 26 = 29
29-26 = 3
The two missing numbers are 55 and 29.
