Chapter 6 Algebra Play (Class 8 - Latest Maths NCERT (Ganita Prakash II) Solutions)

Question 1. Without building the entire pyramid, find the number in the topmost row given the bottom row in each of these cases:

Question 2. Write an expression for the topmost row of a pyramid with $4$ rows in terms of the values in the bottom row.

Question 3. Without building the entire pyramid, find the number in the topmost row given the bottom row in each of these cases:

Recall the Virahāṅka-Fibonacci number sequence $1, 2, 3, 5, \dots$ where each number is the sum of the two numbers before it.

Question 4. If the first three Virahāṅka-Fibonacci numbers are written in the bottom row of a number pyramid with three rows, fill in the rest of the pyramid. What numbers appear in the grid? What is the number at the top? Are they all Virahāṅka-Fibonacci numbers?

Question 5. What can you say about the numbers in the pyramid and the number at the top in the following cases?

(i) The first four Virahāṅka-Fibonacci numbers are written in the bottom row of a four row pyramid.

(ii) The first $29$ Virahāṅka-Fibonacci numbers are written in the bottom row of a $29$ row pyramid.

Question 6. If the bottom row of an $n$ row pyramid contains the first $n$ Virahāṅka-Fibonacci numbers, what can we say about the numbers in the pyramid? What can we say about the number at the top?

Question 1. Fill the digits $1, 3,$ and $7$ in $\square \ \square \times \square$ to make the largest product possible.

Question 2. Fill the digits $3, 5,$ and $9$ in $\square \ \square \times \square$ to make the largest product possible.

Question 1. In the trick given above, what is the quotient when you divide by $9$? Is there a relationship between the two numbers and the quotient?

Question 2. In the trick given above, instead of finding the difference of the two $2$-digit numbers, find their sum. What will happen? For example:

• We start with $31$. After reversing we get $13$. Adding $31$ and $13$, we get $44$.

• We start with $28$. After reversing we get $82$. Adding $28$ and $82$, we get $110$.

• We start with $12$. After reversing we get $21$. Adding $12$ and $21$, we get $33$.

Observe that all these numbers are divisible by $11$. Is this always true? Can we justify this claim using algebra?

Question 3. Consider any $3$-digit number, say $abc$ ($100a + 10b + c$). Make two other $3$-digit numbers from these digits by cycling these digits around, yielding $bca$ and $cab$. Now add the three numbers. Using algebra, justify that the sum is always divisible by $37$. Will it also always be divisible by $3$? [Hint: Look at some multiples of $37$.]

Question 4. Consider any $3$-digit number, say $abc$. Make it a $6$-digit number by repeating the digits, that is $abcabc$. Divide this number by $7$, then by $11$, and finally by $13$. What do you get? Try this with other numbers. Figure out why it works. [Hint: Multiply $7, 11$ and $13$.]

Question 5. There are $3$ shrines, each with a magical pond in the front. If anyone dips flowers into these magical ponds, the number of flowers doubles.

A person has some flowers. He dips them all in the first pond and then places some flowers in shrine $1$. Next, he dips the remaining flowers in the second pond and places some flowers in shrine $2$. Finally, he dips the remaining flowers in the third pond and then places them all in shrine $3$.

If he placed an equal number of flowers in each shrine, how many flowers did he start with? How many flowers did he place in each shrine?

Question 6. A farm has some horses and hens. The total number of heads of these animals is $55$ and the total number of legs is $150$. How many horses and how many hens are on the farm?

Can you solve this without letter-numbers?

[Hint: If all the $55$ animals were hens, then how many legs would there be? Using the difference between this number and $150$, can you find the number of horses?]

Question 7. A mother is $5$ times her daughter’s age. In $6$ years’ time, the mother will be $3$ times her daughter’s age. How old is the daughter now?

Question 8. Two friends, Gauri and Naina, are cowherds. One day, they pass each other on the road with their cows. Gauri says to Naina, “You have twice as many cows as I do”. Naina says, “That’s true, but if I gave you three of my cows, we would each have the same number of cows”. How many cows do Gauri and Naina have?

Question 9. I run a small dosa cart and my expenses are as follows:

• Rent for the dosa cart is $\textsf{₹}5000$ per day.

• The cost of making one dosa (including all the ingredients and fuel) is $\textsf{₹}10$.

(i) If I can sell $100$ dosas a day, what should be the selling price of my dosa to make a profit of $\textsf{₹}2000$?

(ii) If my customers are willing to pay only $\textsf{₹}50$ for a dosa, how many dosas should I aim to sell in a day to make a profit of $\textsf{₹}2000$?

Question 10. Evaluate the following sequence of fractions:

$\frac{1}{3}, \frac{(1+3)}{(5+7)}, \frac{(1+3+5)}{(7+9+11)}$

What do you observe? Can you explain why this happens?

[Hint: Recall what you know about the sum of the first $n$ odd numbers.]