Notice the last digit of the number in each row is making the following pattern
$$ 3, ~ 9, ~ 7, ~ 1, ~ 3, ~ 9, ~ 7, ~ 1, ~ 3, ~ 9, ~ 7, ~ 1 $$
The pattern 3, 9, 7, 1 repeats every four rows. Now count how many times this pattern will repeat in 655 rows. To do that, simply count how many 4-rows are there in 655.
$$ 655 \div 4 = 163 ~ \text{remainder} ~ 3$$
This simply means that there are 163 counts of full { 3, 9, 7, 1 } and the remainder represents 3 more rows. The three remaining rows in the pattern are { 3, 9, 7 }.
$13^1 = 13$
$13^2 = 169$
$13^3 = 2,197$
$13^4 = 28,561$
$13^5 = 371,293$
$13^6 = 4,826,809$
$13^7 = 62,748,517$
$13^8 = 815,730,721$
$13^9 = 10,604,499,373$
$13^{10} = 137,858,491,849$
$13^{11} = 1,792,160,394,037$
$13^{12} = 23,298,085,122,481$
Notice the last digit of the number in each row is making the following pattern
$$ 3, ~ 9, ~ 7, ~ 1, ~ 3, ~ 9, ~ 7, ~ 1, ~ 3, ~ 9, ~ 7, ~ 1 $$
The pattern 3, 9, 7, 1 repeats every four rows. Now count how many times this pattern will repeat in 655 rows. To do that, simply count how many 4-rows are there in 655.
$$ 655 \div 4 = 163 ~ \text{remainder} ~ 3$$
This simply means that there are 163 counts of full { 3, 9, 7, 1 } and the remainder represents 3 more rows. The three remaining rows in the pattern are { 3, 9, 7 }.
Thus, the last digit of 13655 is $7$.
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