What is the unit digit in the multiplication of 1 × 3 × 5 × 7 × 9 × . . . × 999 ?

The product is of all odd numbers from 1 to 999. Since 5 is one of the factors, and the other factors include odd numbers, the product will end in 5 (any odd number multiplied by 5 gives a number ending in 5). Therefore, the unit digit of the entire product is 5.