(I will do the first two parts of each question)
1a. 13 is only divisible by 1 and 13, so they are its factors.
1b. 30 can be divided by 1 and 30, and 2*15=30, 3*10=30, and 5*6=30, so 30's factors are 1, 2, 3, 5, 6, 10, 15, and 30.
2. 75's factors are: 1, 3, 5, 15, 25, and 75. So, there can be 75 piles of 1, 25 piles of 3, 15 piles of 5, etc. (3 more)
3a. 3*1=3, 3*2=6, 3*3=9, etc. so the first 5 multiples of 3 are: 3, 6, 9, 12, 15
3b. Same as before, so: 10, 20, 30, 40, 50
