The lowest base in which the number 987 could be a valid number is 10. The number 987 is a valid number in base 10.
In base 10, also known as the decimal system, we use ten digits from 0 to 9 to represent numbers. Each digit's value is determined by its position, with the rightmost digit representing ones, the next digit representing tens, the following digit representing hundreds, and so on.
The number 987 in base 10 represents 9 hundreds, 8 tens, and 7 ones, which adds up to 900 + 80 + 7 = 987.
The number 987 is a valid number in base 10, which is the standard numbering system we commonly use.