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If the cost of 7 calculators is 4900 rupees, what will be the cost of 15 such calculators?
9500
10500
11500
12500
10500
Observe that 4900 is divisible by 7 giving 700 per unit. Simply calculate 15×700 by multiplying 15×7=105 and adding two zeros.
Cost of 7 calculators = 4900 rupees. We need to find the cost of 15 calculators.
Total Cost=(Number n1Cost of n1)×Number n2
Observe that 4900 is divisible by 7 giving 700 per unit. Simply calculate 15×700 by multiplying 15×7=105 and adding two zeros.
Students often divide 15 by 7 first, which results in a repeating decimal, making the multiplication unnecessarily difficult and prone to rounding errors.
Calculate cost of one unit
Determine the cost of a single calculator by dividing the total cost by the number of units: 4900÷7.
1 calculator=74900=700
Calculate cost of 15 units
Multiply the cost of one unit by the desired number of units: 700×15.
15 calculators=700×15=10500
B is correct because the cost of 15 calculators is 700×15=10500 rupees.
This unitary approach is the foundation for solving Time & Work, Speed-Distance, and Profit & Loss problems where proportionality is constant.