Addition Level 18: 3d + 3d (overflow). answer-window addition. 1. Fill hundreds684 + 547Step 1: Fill hundreds684plus54711raw windows6 + 5 = 11 then keep 1; carry waitsrunning strip11Hundreds window: 6 + 5 = 11. Hold raw 11; carry settles later.2. Fill tensStep 2: Fill tens684plus5471112raw windows8 + 4 = 12 then keep 2; carry waitsrunning strip112Tens window: 8 + 4 = 12. Hold raw 12; settle carry later.3. Fill onesStep 3: Fill ones684plus547111211raw windows4 + 7 = 11 then keep 1; carry waitsrunning strip1121Ones window: 4 + 7 = 11. Hold raw 11; settle carry later.4. Settle carryStep 4: Settle carryKeep ones; carry moves left.raw windows11 | 12 | 11settled digits1 | 2 | 3 | 1answer 1231Settle raw 11 | 12 | 11 into 1231; keep ones and move carry left.