Need help with word problem I am trying to solve this problem. Can someone please help me with the proper solution. My answer is "$2$ hours," but I want a step by step solution I can teach to my kid.

Problem: A tiger eats a deer in $3$ hours and a bear takes $6$ hours to eat it. If they together eat it how much time will they take when tiger eats $2/3$ & beer eats $1/3$. 

 A: For a step-by-step solution, consider this:


*

*The tiger takes $3$ hours to eat $1$ deer. This corresponds to "eating rate" of $\frac{3 \text{ hours}}{1 \text{ deer}}$. 

*The bear takes $6$ hours for $1$ deer. This corresponds to "eating rate" of $\frac{6 \text{ hours}}{1 \text{ deer}}$.
Now, we know that the tiger needs to eat $\frac{2}{3}$ of a deer. This will take
$$
\require{cancel}
\require{xcancel}
\frac{2}{3} \text{ deer}\cdot\frac{3 \text{ hours}}{1 \text{ deer}} = \frac{2}{\xcancel 3} \xcancel{ \text{deer}}\cdot\frac{\xcancel{3} \text{ hours}}{1 \xcancel{\text{ deer}}} = 2 \text{ hours}
$$
For the bear, we know that it needs to eat $\frac{1}{3}$ of a deer. This will take
$$
\require{cancel}
\frac{1}{3} \text{ deer}\cdot\frac{6 \text{ hours}}{1 \text{ deer}} = \frac{1}{\xcancel 3} \xcancel{ \text{deer}}\cdot\frac{2\xcancel{3} \text{ hours}}{1 \xcancel{\text{ deer}}} = 2 \text{ hours}
$$
 Since they both take $2 \text{ hours}$ to eat there respective poritions, we can conclude that the total time will be $2 \text{ hours}$.
This technique of letting units guide you through a problem is known as Dimensional Analysis, which is a very useful to tool develop at a young age for solving problems.
