Can somebody please explain number 15 and number 18 to me, I'm begging you I cannot do this to save my life I've been trying to do it for an hour now

Hey there! I’m happy to help!

NUMBER 15
To solve this problem we want to round to the nearest millionth. We cannot actually write out the entire number because it goes on forever, but we can expand long enough so we can round it the millions place.

One million has six zeroes. This means that a millionth is six decimal places over (0.000001 is one millionth). Same goes for other base ten numbers (100 has two zeroes, one hundredth is two decimal places over)

We see that there is a line over the 45 in our decimal. This means that after this 45 it is going to go 4545454545 forever. We want to round to the nearest millionth. However, when you round, you need the next digit over. Let’s expand our to the ten millionth (seven digits.)
6.2145454
Now, we look at our millionths place. The next place over is four. Therefore, we round down. Therefore, the answer is D) 6.214545

QUESTION 18
Solving questions like this is very simple. With proportions (two equal ratios of fractions) the product of the diagonal sides are equal. So, you just multiply the two numbers that are diagonal from each other and then divide it by the other number and you have your x. If you want, I can explain to you more why this works.

7(2/7)=2
2/2 1/4= 8/9

Therefore, the answer is 8/9.

I hope this has been helpful! If you need anymore help please ask! :D