Write the numbers out using the column method. Start from the right, and simply add the numbers.
111 is 7 if converted back to denary.
Sometimes a binary addition will require you to carry over values into the next highest place-value column, eg when finding the sum of the binary numbers 0010 and 0111:
There is a clash when adding two ones in the same column. In binary, 1+1 is 10 - it has to become 0 with 1 carried over.
1001 is 9 if converted back to denary. 2 + 7 = 9 in denary.