Find the sum and difference between the place values of two sevens in the number 6784037525 and of nine in 49560327 and 1024968.

The first '7' is in the hundred millions place.
Place value of first $7 = 7 \times 100,000,000 = 700,000,000$.

The second '7' is in the thousands place.
Place value of second $7 = 7 \times 1,000 = 7,000$.

The sum is $\boxed{700,007,000}$.

The difference is $\boxed{699,993,000}$.

12. c) Find the sum and difference between the place values of nine in the numbers 49560327 and 1024968.

The '9' is in the millions place.
Place value of '9' in $49,560,327 = 9 \times 1,000,000 = 9,000,000$.

The '9' is in the hundreds place.
Place value of '9' in $1,024,968 = 9 \times 100 = 900$.

The sum is $\boxed{9,000,900}$.

The difference is $\boxed{8,999,100}$.

13. a) In an examination of Mathematics, Himmat got 85 marks and Chhiring got the marks formed by reversing the digits of Himmat's marks.

(i) Find the marks obtained by Chhiring.

The reversed number is 58.
Chhiring's marks are $\boxed{58}$.

(ii) Find the sum and difference of their marks.

The sum of their marks is $\boxed{143}$.

The difference of their marks is $\boxed{27}$.

13. b) In a shop, the cost of an umbrella is Rs 468 and the cost of a school-bag is fixed by interchanging the digits at hundreds and tens place of the cost of the umbrella.

(i) What is the cost of the school-bag?

The new number will have 6 at the hundreds place, 4 at the tens place, and 8 at the ones place.
The cost of the school-bag is Rs $\boxed{648}$.

(ii) How much money do you need to pay to buy both items?

You need to pay Rs $\boxed{1116}$ to buy both items.

(iii) Which item is more expensive and by how much?

Umbrella = Rs 468, School-bag = Rs 648.
The school-bag is more expensive.

The school-bag is more expensive by Rs $\boxed{180}$.