He was not supposed to simplify. He could have multiplied straight across and got 12/63 and he would simplify that which would be 4/21 im thinking
The complete table will be:

<u><em>Explanation</em></u>
Suppose, the number of sit-ups in Sunday is 
Mean is the simple average of some numbers. <u>For finding the mean number</u> of sit-ups per day, we need to <u>divide the 'total number of sit-ups' by the 'total number of days'</u>.
Here the total number of days is 7 and the total number of sit-ups for 7 days 
Given that, the mean number of sit-ups per day is 100. So, the equation will be...

So, the number of sit-ups you have to do Sunday is 175.
1 over 2 equals 2 over 4 equals 4 over 8.
Hope this helps! :)
Answer:
See the explanation below.
Step-by-step explanation:

Starting from 
By putting
we have:

I think it might be pairs 2 & 3