7^1= Units is 7
7^2= Units is 9
7^3= Units is 3
7^4= Units is 1
7^5= Units is 7.....incremental powers after 4th power of 7, the units digit is repeated.
177^28 => Units digit = (7^4)^7=7^28. Therefore Units digits is 1
3^1= Units is 3
3^2= Units is 9
3^3= Units is 7
3^4= Units is 1
3^5= Units is 3.....incremental powers after 4th power of 3, the units digit is repeated.
133^23 => Units digit = 133^20+3 . Units digits of 133^20 is 1 but we need ^23. So multiply 3 times which means units digit of 3^3 which is 7
Combining the two and taking difference of units digit. 1 - 7. Borrow one from whatever tens place which equals 11-7 = 4
Answer is C
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