Maths : Basic Numeracy

Q 165 / 370

UPSC CSE Prelims 2021

Consider the following multiplication problem:
(PQ) × 3 = RQQ, where P, Q and R are different digits and R ≠ 0.
What is the value of (P + R) + Q?

EXPLANATION

Correct Option (B)

The given multiplication problem is (PQ) × 3 = RQQ, where P, Q, and R are distinct digits and R ≠ 0.

This can be expressed algebraically:

  • The two-digit number PQ represents 10P+Q.
  • The three-digit number RQQ represents 100R+10Q+Q=100R+11Q.

Substituting these into the given equation:

(10P+Q)×3=100R+11Q

30P+3Q=100R+11Q

Rearranging the terms to simplify the equation:

30P=100R+8Q

From the equation 30P=100R+8Q, the last digit of 30P is 0, and the last digit of 100R is also 0. Consequently, the last digit of 8Q must be 0.

For 8Q to have a last digit of 0, and Q being a single digit, Q must be 5 (since 8×5=40). Q cannot be 0, as PQ×3=RQQ would imply P0×3=R00, which is not possible for distinct digits P, Q, R and R ≠ 0 (e.g., 10×3=30, not R00).

Substitute Q=5 back into the equation 30P=100R+8Q:

30P=100R+8(5)

30P=100R+40

Divide the entire equation by 10 to simplify:

3P=10R+4

Now, test integer values for R (where R ≠ 0 and R is a single digit) to find corresponding integer values for P:

  • If R=1, 3P=10(1)+4=14⇒P=143 (not an integer).
  • If R=2, 3P=10(2)+4=24⇒P=8 (an integer).
  • If R=3, 3P=10(3)+4=34⇒P=343 (not an integer).
  • If R=4, 3P=10(4)+4=44⇒P=443 (not an integer).
  • If R=5, 3P=10(5)+4=54⇒P=18 (not a single digit).

The only valid solution for P and R as distinct single digits (with R ≠ 0) is P=8 and R=2.

Thus, the determined digits are P=8, Q=5, and R=2.

Verification: 85×3=255, which matches the RQQ format.

The problem asks for the value of (P+R)+Q. However, to align with the provided options and the solution's final calculation, it is interpreted as P+RQ.

Calculating the value: P+RQ=8+25=105=2.

Therefore, the correct value is 2.

Incorrect Options:

Options A (1), C (5), and D (insufficient data) are incorrect. A unique set of values for P, Q, and R was determined through systematic algebraic deduction and testing. This allowed for the calculation of the expression P+RQ, yielding a specific numerical result of 2. Therefore, the data is sufficient, and the other numerical options do not match the derived value.