Carla adds together the multiples of 3, in order and tell her friends to guess which number was the last one she added if her total is 84

28

Step-by-step explanation:

I know that 20 times 3 equals 60 and that 24 plus 60 equals 84 so we need to find out what times 3 equals 24 I know that 8 times 3 equals 24 so we add 20 and 8 to get our answer 28