If x = 2 , we have x2 + 2x = 8 ( scrap because 8 isn't a prime number)

If x = 3 , we have x2 + 2x = 17 ( satisfy )

If x > 3 and x is a prime number then x =3k + 1 or 3k + 2 ( k is a natural number)

+) x = 3k +1

If x = 3k +1 then x2 divide to 3 has the remainder 1

and 2x divide to 3 has the remainder 2

=> 2x + x2 divided to 3 

But 2x + x2 > 3 

So 2x + x2 isn't a prime number

Do the similar to x = 3k+2

The answer is x = 3

If x = 2 , we have x2 + 2x = 8 ( scrap because 8 isn't a prime number)

If x = 3 , we have x2 + 2x = 17 ( satisfy )

If x > 3 and x is a prime number then x =3k + 1 or 3k + 2 ( k is a natural number)

+) x = 3k +1

If x = 3k +1 then x2 divide to 3 has the remainder 1

and 2x divide to 3 has the remainder 2

=> 2x + x2 divided to 3 

But 2x + x2 > 3 

So 2x + x2 isn't a prime number

Do the similar to x = 3k+2

The answer is x = 3

A notebook costs: 16.80 : 8 = 2.10 ($)

A pen costs: 15.25 : 5 = 3.05 ($)

Anna buys: 2.10 x 4 + 3.05 x 8 = 8.40 + 24.40 = 32.80 ($) 

Anna received: 2 x 20 - 32.80 = 7.20 ($)

Benny buys: 5 x 2.10 + 10 x 3.05 = 10.50 + 30.50 = 41 ($)

Benny received: 2 x 20 - 41 = -1 ($) 

=> Anna received more and more than: 7.20 + 1 = 8.20 ($)