Take a look at the math picture

Question: (4 x 10⁴)(2 x 10¹)

Using the knowledge that a x 10ⁿ = a followed by ⁿ zeroes (eg: 5 x 10³ = 5,000)

We know that:

4 x 10⁴ is equal to 40,000

2 x 10¹ is equal to 20

Using the knowledge that (a)(b) = a x b, our next step would be to multiply those figures together.

40,000 x 20. We can do this without a calculator by counting how many zeroes are there.

Since there’s 5 zeroes, we know our answer will have 5 zeroes and we can take those 5 zeroes out of our equation:

So instead of 40000 x 20, we have 4 x 2 which we know is 8. Using our knowledge that our answer will have 5 zeroes, 40000 x 20 is equal to 800,000 (8 followed by 5 zeroes).

Our final step is to put that number into scientific notation. We know that 800,000 contains 5 zeroes, therefore 800,000 in scientific notation is:

8 x 10⁵

The correct answer would be D.

I hope this made sense and helps you and if you have any questions or think that I’ve made a mistake please let me know!

:)