Assume the boy multiplied 987 with number K and the coreect answer is 55ab81.
Kx987 = 550000 + ax1000 + bx100 + 81
K = 557 + (241 + ax1000 + 100b +81)/987
K- 557 = (322 + ax1000 + bx100)/987
(322 + ax1000 + bx100)/987 = n
Check for a =0 , b=0,1,2...9
a =1 , b=0,1,2...9
a =2 , b=0,1,2...9
a =3 , b=0,1,2...9
a =4 , b=0,1,2...9
a =5 , b=0,1,2...9
a = 6 , b = 4
So the correct answer is 556481.