+25 Compute 997−1 modulo 1000. Express your answer as an integer from 0 to 999.
+150 1997≡x(mod1000)
997|1000(1000-a)+1
(1000-3)|(1000)(1000-a)+1
last 3 digits from (1000)1000-a+1 = 001.
then, last 3 digits from 1000-3 is 001 too.
chance : (1000-3)*333.
=> 997*333.
x=333
