The highest common factor of 10 and 15 is _____. 

10 can be written as 10 = $1\times 2\times 5$

15 can be written as 15 = $1\times 3\times 5$

The product of all common prime factors is the HCF.

The common prime factors of 10 and 15 are 5 and 1.

Therefore, the highest common factor of 10 and 15 is 5.