If you think about it as 5/5 is 1 and one load is 1/5, you need 5 loads to get 1 ton. Then, you need another 3. To get this you do 5 X 3. So you need 15 loads to get 3. Then add 15 + 5 which then gives you 20
You can multiply by 12 because to get rid of the fractions all terms should be divided by the LCM (ie the smallest multiple that is exactly divisible by 2, 4 and 3)
Multiples of 3 = 3, 6, 9, 12
Multiples of 4 = 4, 8, 12, 16
Multiples of 2 = 2, 4, 6, 8, 10, 12
Therefore the fractions should be multiplied by 12