So the question I believe is:
In a counting system used by intelligent apes. A banana = 1. 6 is
represented by an orange and 2 bananas An orange is worth half a mango
What is the value of two mangos, an orange and a banana?
So you need to work out the value of each fruit by itself. A banana is given to you as being 1. Next, an orange plus two bananas is 6. We already know that a banana is 1 and there are two of them here so 6 - 2 = 4. An orange is 4. Now they tell us an orange (4) is worth half a mango. So a mango must be 8. So that an orange can be worth half of it.
Now the question is
What is the value of two mangos, an orange and a banana?
Banana = 1 orange = 4 mango = 8
8 + 8 + 4 + 1 = 21
And the next question for bonus is: What is two mangos and an orange,
divided by an orange with a banana?
An orange
(8 + 8 + 4) / (4 + 1) = 4 = orange
:)