WAEC Mathematics 2010 Paper

Concept:Subtraction in base seven works like ordinary subtraction, but when you borrow, $1$ from the next column equals $7$ in the current column.Explanation:Subtract from right to left, column by column.In the units column: $2 - 4$ is not possible, so borrow $1$ from the tens column. The borrowed $1$ becomes $7$ in the units column, giving $2 + 7 = 9$. Then $9 - 4 = 5$.The top tens digit $6$ becomes $5$ after this borrow. In the tens column: $5 - 4 = 1$. This matches the shown tens digit $1$.In the hundreds column, the top digit is $1$ and the bottom digit is $6$. Since $1 < 6$, borrow $1$ from the thousands column. The borrowed $1$ becomes $7$ in the hundreds column, so $1 + 7 = 8$. Then $8 - 6 = 2$.Therefore the missing digit is $2$. After this borrow, the thousands column gives $4 - 2 = 2$, so the result is $2\,2\,1\,5_{\text{seven}}$.Answer:$2$ (Option A)