Solve Using Reference Equation: Finding 39+39 Using 40+40=80

$40+40=80$

Use the above equation to help solve the following problem:

$39+39=\text{?}$

To solve this problem, we'll use the information that $40 + 40 = 80$. We'll employ the following steps:

• Step 1: Realize that $39$ is 1 less than $40$. Therefore, $39 + 39$ can be rewritten using the relationship $40 + 40$ as $(40 - 1) + (40 - 1)$.
• Step 2: Simplify the expression to calculate the total: $(40 + 40) - (1 + 1) = 80 - 2$.
• Step 3: Subtract 2 from 80 to obtain the result.

Now, let's perform the calculation:

$40 + 40 = 80$ implies that $39 + 39 = 80 - 2 = 78$.

Therefore, the solution to the problem is $78$.