Hey Mate,

I will answer your questions part-wise.

7 ) a )

Given,

The weight of 72 books is 9 kg.

Then, if we take 1 book to be "x". Then,

=> 72x = 9 kg.

On transposing 72 from LHS to RHS, we get:-

=> x = 9/72 kg

Now, we get the weight of 1 book to be 9/72 kg.

But since, we have to find the weight of 80 books, we will have to multiply the weight of one book, i.e., (9/72 ) by 80.

Weight of 80 books = (( 9/72 ) * 80 ) kg

Weight of 80 books = ( 720/72 ) kg

Weight of 80 books = 10 kg.

Therefore, the weight of 80 books is equal to 10 kg.

7 ) b )

Given,

The weight of 72 books is 9 kg.

Then, if we take 1 book to be "x". Then,

72x = 9 kg

On transposing 9 from LHS to RHS, we get:-

= 1 kg

Now, we get to know that 8 books measures 1 kg.

But since, we have to find the number of books that will weigh 6 kg, we will have to multiply the number of  books that measures 1 kg by 6, i.e., ( 8 ) by 6.

6 kg = ( 8 ) * 6

6 kg =  48 books

Therefore, 48 books will weigh 6 kg.

Hope it Helps!