Answer: 102
Started:
81, 81+90 = 171, 171 + 83 = 254, 254+ 94 = 348, 348+ 82 = 430, 430 +108 = 538
081
171
254
348
430
538
XXX
Isolate the hundreds and tens digits and take the first difference
08 > 17 > 25 > 34 > 43 > 53 > XXXX
9 8 9 9 10
-1 1 0 1 *second difference
apply fibonanci
-1, 1,0,1,1
for second difference to be one first difference =11 >> 53 + 11 = 64
making hundreds and tens digits have to be 64 only way to get there is 538 + 102 = 640