OK, let's crack this. We're given 780 and our task is to multiply it by 5. Essentially, we'll be adding 780 to itself 5 times.
Step 1: 0 + 780 = 780
Step 2: 780 + 780 = 1560
Step 3: 1560 + 780 = 2340
Step 4: 2340 + 780 = 3120
Step 5: 3120 + 780 = 3900

So, 780*5 = 3900