Unit digit in `7^4` is 1. `:.` Unit digit in `7^68` is 1. `:.` Unit digit in `7^71` is 3. [`1 xx 7 xx 7 xx 7` gives unit digit 3] Again, every power of 6 will give unit digit 6. `:.` Unit digit in `6^59` is 6. Clearly, unit digit in `3^4` is 1. So, unit digit in `3^64` is 1. `:.` Unit digit in `3^65` is 3. `:.` Unit digit in `(7^71 xx 6^59 xx 3^65)` is 4. [`3 xx 6 xx 3` gives unit digit 4]