a. ( 800 - 70 × 10 + 64 ) × 10 = 1640
= ( 800 - 700 + 64 ) × 10
= ( 100 + 64 ) × 10
= 164 × 10
= 1640
b. ( 50 : 10 + 400 : 2 - 165 ) : 10 + 999 = 1003
= ( 5 + 400 : 2 - 165 ) : 10 + 999
= ( 5 + 200 - 165 ) : 10 + 999
= ( 205 - 165 ) : 10 + 999
= 40 : 10 + 999
= 4 + 999
= 1003