Sunita buys 5 kg of rice at Rs 90 per kg and 2 kg of sugar at Rs 70 per kg. Write a mathematical expression and simplify it to find the total cost and the change she gets back if she pays with Rs 1000

Cost of rice $= 5 \text{ kg} \times \text{Rs } 90/\text{kg} = \text{Rs } 450$

Cost of sugar $= 2 \text{ kg} \times \text{Rs } 70/\text{kg} = \text{Rs } 140$

Total cost $= (5 \times 90) + (2 \times 70)$
Total cost $= 450 + 140 = 590$
The total cost is Rs 590.

Change $= 1000 - 590 = 410$
The shopkeeper returned Rs 410.

d) On a day, there were 27 students present in class six. 16 of them are girls and the rest were boys. If only 3 boys were absent on that day, represent the information by a mathematical expression and find the number of boys in class six.

Total boys present $= 27 - 16 = 11$

Total boys in class $= (27 - 16) + 3$
Total boys in class $= 11 + 3 = 14$
The number of boys in class six is 14.

9. a) Let's make any four your own mixed expressions using all $(+, -, \times, \div)$ in each expression. Simplify them and get answers.

Expression 1:

Expression 2:

Expression 3:

Expression 4:

9. b) Let's rewrite these simplifications and find the mistakes. Then, show the simplification in the correct way.

Box 1:
Given:

Mistake: Addition was performed before subtraction. Operations of the same precedence (addition and subtraction) should be done from left to right.
Correct simplification:

Box 2:
Given:

Mistake: The second subtraction ($6-4$) was performed before the first subtraction ($27-6$). Subtraction operations should be done from left to right.
Correct simplification:

Box 3:
Given:

Mistake: Addition was performed before multiplication. Multiplication has higher precedence than addition.
Correct simplification:

Box 4:
Given:

Mistake: Subtraction was performed before multiplication. Multiplication has higher precedence than subtraction.
Correct simplification:

Box 5: (Based on the visible part, assuming the expression is $40 \div 5 + 3$)
Given:

This simplification is correct. Division was performed before addition, following the order of operations.