Example 3 - Chapter 4 Class 7 - Expressions using Letter-Numbers (Ganita Prakash)

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Example 3 Ketaki prepares and supplies coconut-jaggery laddus. The price of a coconut is ₹35 and the price of 1 kg jaggery is ₹60. (a) How much should she pay if she buys 10 coconuts and 5 kg jaggery? (b) How much should she pay if she buys 8 coconuts and 9 kg jaggery? (c) Write an algebraic expression to find the total amount to be paid for a given number of coconuts and quantity of jaggery. (d) Use this expression (or formula) to find the total amount to be paid for 7 coconuts and 4 kg jaggery.Given Cost of coconut = ₹ 35 Cost of 1kg jaggery = ₹ 60 (a) How much should she pay if she buys 10 coconuts and 5 kg jaggery? Cost of 10 coconuts = 10 × Cost of 1 coconut = 10 × ₹ 35 = ₹ 350 Cost of 5 kg jaggery = 5 × Cost of 1 kg jaggery = 5 × ₹ 60 = ₹ 300 Thus, Total cost = 350 + 300 = ₹ 650 (b) How much should she pay if she buys 8 coconuts and 9 kg jaggery? Cost of 8 coconuts = 8 × Cost of 1 coconut = 8 × ₹ 35 = ₹ 280 Cost of 5 kg jaggery = 9 × Cost of 1 kg jaggery = 9 × ₹ 60 = ₹ 540 Thus, Total cost = 280 + 540 = ₹ 820 (c) Write an algebraic expression to find the total amount to be paid for a given number of coconuts and quantity of jaggery. To make algebraic expressions, we have to figure out variables Let’s assume Number of coconuts = c Quantity of jaggery in kgs = j Thus, Cost of c coconuts = c × Cost of 1 coconut = c × 35 = 35c Cost of j kg jaggery = j × Cost of 1 kg jaggery = j × 60 = 60j Thus, Total cost = 35c + 60j Note: We wrote 35 × c as 35c and 60 × j as 60j as it is easier to write We can also write Total cost = 35 × c + 60 × j (d) Use this expression (or formula) to find the total amount to be paid for 7 coconuts and 4 kg jaggery Now, our expression is Total cost = 35c + 60j Where Number of coconuts = c Quantity of jaggery in kgs = j We have 7 coconuts and 4 kg jaggery Thus, c = 7 and j = 4 Finding total cost Total cost = 35c + 60j = 245 + 240 = ₹ 485