Let a represent the red spheres and b represent the blue spheres. write a balanced equation for the equilibrium reaction.

I have attached the diagram related to this question.

5A₂ + 5B .................> 4A₂B + A₂ + B

Explanation:
1- For the reactants:
We can note that:
i- we have 5 moles of substance A which is diatomic. This means that we have 5A₂
ii- we have 5 moles of substance B which is monoatomic. This means that we have 5B
Based on the above, for the reactants side, we have:
5A₂ + 5B

2- For the products:
We can note that:
i- we have 4 moles of a substance formed from the combination of two moles of substance A with one mole of substance B. This means that we have 4A₂B
ii- we have one mole of substance A that is diatomic. This means that we have A₂
iii- we have one mole of substance B that is monoatomic. This means that we have B
Based on the above, for the products side, we have:
4A₂B + A₂ + B

3- Combining reactants and products parts:
Combining the reactants side (from part 1) with the products side (from part 2), we can find that the initial equation is as follows:
5A₂ + 5B .................> 4A₂B + A₂ + B

4- Balancing the equation:
Taking a look at the initial equation we reached in part 3, we will find that:
Number of moles of substance A in reactants = 2*5 = 10 moles
Number of moles of substance A in products = 2*4 + 2*! = 10 moles
SUBSTANCE A IS BALANCED
Number of moles of substance B in reactants = 5*1 = 5 moles
Number of moles of substance B in products = 4*1 + 1 = 5 moles
SUBSTANCE B IS BALANCED
Based on the above, the final balanced equation is the same as the initial equation reached in part 3:
5A₂ + 5B .................> 4A₂B + A₂ + B

Hope this helps :)

Adozen is 1221/12 = 1.751.75 x 12 = 21therefore, 1.75 is the answer.