Maths : Basic Numeracy

Q 362 / 370

UPSC CSE Prelims 2011

A person has only ₹1 and ₹2 coins with her. If the total number of coins that she has is 50 and the amount of money with her is ₹75, then the number of ₹1 and ₹2 coins are, respectively

EXPLANATION

Correct Option (4)

Let 'x' represent the number of ₹1 coins and 'y' represent the number of ₹2 coins.

Based on the problem statement, two linear equations can be formulated:

  • Total number of coins: x + y = 50
  • Total amount of money: 1x + 2y = 75

To solve this system of equations:

  1. From the first equation, express x in terms of y: x = 50 - y
  2. Substitute this expression for x into the second equation: 1(50 - y) + 2y = 75
  3. Simplify and solve for y: 50 - y + 2y = 75 which simplifies to 50 + y = 75. Therefore, y = 75 - 50 = 25.
  4. Substitute the value of y back into the expression for x: x = 50 - 25 = 25.

Thus, the number of ₹1 coins is 25, and the number of ₹2 coins is 25. This matches option 4.

Incorrect Options:

The other options do not satisfy both conditions (total number of coins = 50 and total amount = ₹75).

  • Option 1 (15 and 35):
    • Total coins: 15 + 35 = 50 (Satisfied)
    • Total amount: 1 * 15 + 2 * 35 = 15 + 70 = 85 (Does not equal ₹75)
  • Option 2 (35 and 15):
    • Total coins: 35 + 15 = 50 (Satisfied)
    • Total amount: 1 * 35 + 2 * 15 = 35 + 30 = 65 (Does not equal ₹75)
  • Option 3 (30 and 20):
    • Total coins: 30 + 20 = 50 (Satisfied)
    • Total amount: 1 * 30 + 2 * 20 = 30 + 40 = 70 (Does not equal ₹75)