OK, let's work through this together. We're starting with 405 and we're going to multiply it by 10, which means we add 405 to itself 10 times.
Step 1: 0 + 405 = 405
Step 2: 405 + 405 = 810
Step 3: 810 + 405 = 1215
Step 4: 1215 + 405 = 1620
Step 5: 1620 + 405 = 2025
Step 6: 2025 + 405 = 2430
Step 7: 2430 + 405 = 2835
Step 8: 2835 + 405 = 3240
Step 9: 3240 + 405 = 3645
Step 10: 3645 + 405 = 4050

So, 405*10 = 4050