OK, let's work through this together. We're starting with 768 and we're going to multiply it by 20, which means we add 768 to itself 20 times.
Step 1: 0 + 768 = 768
Step 2: 768 + 768 = 1536
Step 3: 1536 + 768 = 2304
Step 4: 2304 + 768 = 3072
Step 5: 3072 + 768 = 3840
Step 6: 3840 + 768 = 4608
Step 7: 4608 + 768 = 5376
Step 8: 5376 + 768 = 6144
Step 9: 6144 + 768 = 6912
Step 10: 6912 + 768 = 7680
Step 11: 7680 + 768 = 8448
Step 12: 8448 + 768 = 9216
Step 13: 9216 + 768 = 9984
Step 14: 9984 + 768 = 10752
Step 15: 10752 + 768 = 11520
Step 16: 11520 + 768 = 12288
Step 17: 12288 + 768 = 13056
Step 18: 13056 + 768 = 13824
Step 19: 13824 + 768 = 14592
Step 20: 14592 + 768 = 15360

So, 768*20 = 15360