The answer is 85 miles/hour. Hope it helps! If you could vote me brainiest, that would be awesome, thanks!
From the definition of logarithm:

if

(1)
The equation in our problem is:

If we use definition (1), this equation becomes

Let's solve it:




therefore, the correct answer is C).
So for decimals, add the number of zeroes after the whole number to equal the amount of digits in the decimal value. For example, if you had 10 and 0.5 you would replace 10 with 10.0 to match the amount of digits after the decimal in 0.5
In your problem since we have four digits after the decimal, let's add a decimal and four digits after the number 15 to match it up. Now it should look like the image attached.
Once matched up you can now safely subtract.
Hope that helps!
Please comment on this answer and give me a full question and I will be just fine answering it for you.