If m and n are positive integers such that m + n = 9, what is the valu

1. 39< 3m + 5n < 43

this can be simplified as 39< 3(m + n) + 2n < 43, which gives 39<27 + 2n <43 so, 12<2n<16 or 6<n<8.

Clearly, n = 7. so m + 7 = 9, m = 2.

Thus 1). Sufficient.

2. 8 <= mn <= 18

this doesnt give us any concrete value for mn on its own, therefore insufficient.

Ans is (A)..


Kudos if it helped.

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

Since we have 2 variable (m and n) and 1 equations, D is most likely to be the answer. So, we should consider each of the conditions on their own first.

Condition 1)
39 < 3m + 5n < 43
⇔ 39 < 3m + 3n + 2n < 43
⇔ 39 < 3( m + n ) + 2n < 43
⇔ 39 < 3*9 + 2n < 43
⇔ 39 < 27 + 2n < 43
⇔ 12 < 2n < 16
⇔ 6 < n < 8
⇔ n = 7, since n is an integer.
Condition 1) is sufficient.

Condition 2)
If m = 3, n = 6, then mn = 18.
If m = 2, n = 7, then mn = 14.
Since we don't have a unique solution, condition 2) is not sufficient.

Therefore, A is the answer.

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A,B,C or E.

(1) \(39 < 3m + 5n < 43\)

\(39 < 3(m+n) + 2n < 43\)

\(39 < 27 + 2n < 43\)

\(12 < 2n < 16\)

\(6 < n < 8\)

n = 7; m = 2.

mn = 14

SUFFICIENT.

(2) 8 <= mn <= 18
mn can take multiple values; INSUFFICIENT.

Answer is A.