Whenever a problem required subtraction from 0, he simply flipped the two numbers in the column with the 0. For example, in the problem 307 - 182, he treated 0 - 8 as 8 - 0, and wrote "8" as the answer. The boy's second bug involved not decrementing the number to the left of the zero (not reducing the 3 to 2 in 307 - 182).