Simplify the Expression: 7.8 + 3.5a - 80b - 7.8b + 3.9a

To solve this problem, we need to simplify the given expression:

1. Identify and group similar terms:

• Constant term: $7.8$
• Terms with $a$: $3.5a + 3.9a$
• Terms with $b$: $-80b - 7.8b$

2. Simplify each group:

• Combine terms with $a$:
  $3.5a + 3.9a = (3.5 + 3.9)a = 7.4a$
• Combine terms with $b$:
  $-80b - 7.8b = (-80 - 7.8)b = -87.8b$

3. The constant term remains $7.8$.

4. Therefore, the simplified expression is:
$7.8 + 7.4a - 87.8b$

Thus, the correct answer is $\boxed{7.8 + 7.4a - 87.8b}$