The 4th factor would be 3. You would find this out through a factor tree. If you don't know the original number when you start you look at your numbers. A factor of 1 usually goes with the highest number, assuming that the number 6 is the highest factor, that would be the original number. As 2 is the other known factor than we know that 3 has to be the last factor, because 2 x 3 = 6. It cannot be the number 12 because then there would be 6 factors not the needed 4.
Answer:
3x - (15 x 3) = 90
or
3x - 45 = 90
Explanation:
there are 3 boxes. We don't know how many are in them yet, so we write this as 3x. However, we know that we give away 15 to 3 different teachers. 15 x 3 = 45
Therefore the first part is 3x - 45
We also know that in the end we have 90 left.
So our final equation is:
3x - 45 = 90
