The highest common factor of 21 and 90 is _________.

We know that the highest number that exactly divides the given numbers with a Remainder equal to zero, is called the Highest Common Factor (HCF). 

Now, 21 can be written as 3 × 7 × 1 and,

90 can be written as 3 × 3 × 2 × 5.

HCF, in terms of prime factors, can also be described as the product of all common prime factors of the numbers, when the lower power of each common factor is considered.

Therefore, HCF = 3 × 1 = 3