Simplify each problem and express the answer in scientific notation.

Let's solve each problem step by step, simplifying the expressions and expressing the answers in scientific notation.

---

1) $ (7 \times 10^4)(2 \times 10^5) $

Step 1: Multiply the coefficients:
$ 7 \times 2 = 14 $

Step 2: Add the exponents:
$ 10^4 \times 10^5 = 10^{4+5} = 10^9 $

So we have:
$ 14 \times 10^9 $

But this is not in proper scientific notation (since 14 > 10). Adjust it:

$ 14 \times 10^9 = 1.4 \times 10^{10} $

✔ Answer: $ \boxed{1.4 \times 10^{10}} $

---

2) $ (9 \times 10^3) + (5 \times 10^3) $

Same exponent → add coefficients:

$ 9 + 5 = 14 $, so:

$ 14 \times 10^3 $

Convert to scientific notation:

$ 14 \times 10^3 = 1.4 \times 10^4 $

✔ Answer: $ \boxed{1.4 \times 10^4} $

---

3) $ (3 \times 10^8) - (7 \times 10^6) $

Different exponents → convert both to same power of 10.

Note: $ 3 \times 10^8 = 300 \times 10^6 $

Now subtract:

$ 300 \times 10^6 - 7 \times 10^6 = (300 - 7) \times 10^6 = 293 \times 10^6 $

Convert to scientific notation:

$ 293 \times 10^6 = 2.93 \times 10^8 $

✔ Answer: $ \boxed{2.93 \times 10^8} $

---

4) $ \frac{5 \times 10^8}{4 \times 10^3} $

Divide coefficients:
$ \frac{5}{4} = 1.25 $

Subtract exponents:
$ 10^{8-3} = 10^5 $

So:
$ 1.25 \times 10^5 $

✔ Answer: $ \boxed{1.25 \times 10^5} $

---

5) $ \frac{2 \times 10^4}{8 \times 10^2} $

Divide coefficients:
$ \frac{2}{8} = 0.25 $

Subtract exponents:
$ 10^{4-2} = 10^2 $

So:
$ 0.25 \times 10^2 $

Now adjust to scientific notation:
$ 0.25 \times 10^2 = 2.5 \times 10^{-1} \times 10^2 = 2.5 \times 10^{1} $

Or directly:
$ 0.25 \times 10^2 = 25 $, which is $ 2.5 \times 10^1 $

✔ Answer: $ \boxed{2.5 \times 10^1} $

---

6) $ (2 \times 10^7) - (4 \times 10^5) $

Convert both to same exponent.

$ 2 \times 10^7 = 200 \times 10^5 $

Now subtract:

$ 200 \times 10^5 - 4 \times 10^5 = 196 \times 10^5 $

Convert to scientific notation:

$ 196 \times 10^5 = 1.96 \times 10^7 $

✔ Answer: $ \boxed{1.96 \times 10^7} $

---

7) $ (6 \times 10^4) + (1 \times 10^5) $

Convert to same exponent.

$ 1 \times 10^5 = 10 \times 10^4 $

Now add:

$ 6 \times 10^4 + 10 \times 10^4 = 16 \times 10^4 $

Convert to scientific notation:

$ 16 \times 10^4 = 1.6 \times 10^5 $

✔ Answer: $ \boxed{1.6 \times 10^5} $

---

8) $ (8 \times 10^5)(6 \times 10^7) $

Multiply coefficients:
$ 8 \times 6 = 48 $

Add exponents:
$ 10^{5+7} = 10^{12} $

So:
$ 48 \times 10^{12} $

Adjust to scientific notation:
$ 48 \times 10^{12} = 4.8 \times 10^{13} $

✔ Answer: $ \boxed{4.8 \times 10^{13}} $

---

9) $ (4 \times 10^8)(9 \times 10^5) $

Multiply coefficients:
$ 4 \times 9 = 36 $

Add exponents:
$ 10^{8+5} = 10^{13} $

So:
$ 36 \times 10^{13} $

Adjust:
$ 36 \times 10^{13} = 3.6 \times 10^{14} $

✔ Answer: $ \boxed{3.6 \times 10^{14}} $

---

10) $ (3 \times 10^6) + (7 \times 10^7) $

Convert to same exponent.

$ 3 \times 10^6 = 0.3 \times 10^7 $

Now add:

$ 0.3 \times 10^7 + 7 \times 10^7 = 7.3 \times 10^7 $

✔ Answer: $ \boxed{7.3 \times 10^7} $

---

✔ Final Answers:

1) $ 1.4 \times 10^{10} $
2) $ 1.4 \times 10^4 $
3) $ 2.93 \times 10^8 $
4) $ 1.25 \times 10^5 $
5) $ 2.5 \times 10^1 $
6) $ 1.96 \times 10^7 $
7) $ 1.6 \times 10^5 $
8) $ 4.8 \times 10^{13} $
9) $ 3.6 \times 10^{14} $
10) $ 7.3 \times 10^7 $

Let me know if you'd like a printable version or explanation for any specific step!