Subject: General Aptitude · Chapter: Number System · Exam: · Marks: · Difficulty:
The digit in the unit's place of the number $(67)^{25}-1$ must be
(a)0
(b)6
(c)8
(d)None of these
Answer
Answer (as printed): B
Explanation
Unit digit of $(67)^{25}=$ Unit digit of $7^{25}$. Unit digit of $7^{4}$ is 1 and so the unit digit of $\left(7^{4}\right)^{6}$ is 1 . ∴ Unit digit of $7^{25}=(1 \times 7)=7$. Hence, the unit digit of $\left(7^{25}-1\right)$ is $(7-1)=6$.
Explanation as extracted from the printed page; notation may be imperfect.