A factor of a number is a whole number that divides the given number without leaving a remainder. In other words, if you can divide a number 𝐴 by another number 𝐵 and the result is an integer, then 𝐵 is a factor of 𝐴.
Let's test each option:
90 ÷ 15 = 6 (No remainder, therefore 15 is a factor)
90 ÷ 5 = 18 (No remainder, therefore 5 is a factor)
90 ÷ 6 = 15 (No remainder, therefore 6 is a factor)
90 ÷ 12 = 7.5 (Leaves a remainder of 0.5)
Therefore, 12 is not a factor of 90.