To solve this problem step by step, we need to carefully analyze the equations given and work out the values of the variables $A$, $B$, and $C$, then apply them to find the value of $A+B×C$.

### Step 1: Solve for $A$

We start with the first equation:

$A+A=20$

This simplifies to:

$2A=20$

By dividing both sides by 2:

$A=10$

### Step 2: Solve for $B$

Next, we move to the second equation:

$B+B=10$

This simplifies to:

$2B=10$

By dividing both sides by 2:

$B=5$

### Step 3: Solve for $C$

Now, we solve for $C$ using the third equation:

$C+C=8$

This simplifies to:

$2C=8$

By dividing both sides by 2:

$C=4$

### Step 4: Apply the values to the final equation

Now that we have the values for $A$, $B$, and $C$, we can substitute them into the final equation $A+B×C$:

$A+B×C$

Using the values $A=10$, $B=5$, and $C=4$:

$10+5×4$

According to the order of operations (PEMDAS/BODMAS), multiplication comes before addition, so we first multiply:

$5×4=20$

Now we add:

$10+20=30$

### Final Answer:

The solution to $A+B×C$ is:

$30$

### Explanation Summary:

- $A$ is found by solving $A+A=20$, resulting in $A=10$.
- $B$ is found by solving $B+B=10$, resulting in $B=5$.
- $C$ is found by solving $C+C=8$, resulting in $C=4$.
- Substituting these values into $A+B×C$ and applying the order of operations leads to the final answer of 30.