30. Think of a single-digit number: double it, add 3, multiply by 5, add 7, using the last digit of the resulting number, write down a single-digit number, add 18 to it, and divide the result by 5. What number did you get? No matter what number you thought of, you will always get the same number in the final result. Explain why.
30. Let the number thought of be $x$. Then we sequentially obtain:
1) $2 x ; 2) 2 x+3 ; 3) 10 x+15=10(x+1)+5$
2) $10(x+1)+12 ; 5) 2 ; 6) 2+18=20$; 7) $20: 5=4$.