Question

If $a$, $b$, $c$, and $d$ are replaced by four distinct digits from $1$ to $9$, inclusive, then what's the largest possible value of the difference $a.b - c.d$ ?

Answer 1

Answer:

70

Step-by-step explanation:

• for the value of difference to be largest, the minuend should be maximum(most possibly) and the subtrahend should be minimum

[in A-B=X, A is minuend and B is subtrahend ]

• so, $a.b should be maximum. as there is a condition that 4 digits should be distinct, the product will be maximum if we choose 2 maximum valued numbers from the given numbers. so, one of them should be 9 and the other should be 8.

therefore, $a.b=9*8=72

• as mentioned above, c.d$ should be minimum. this will be possible only when we choose  2 minimum valued numbers from the given numbers. so, one of them should be 1 and the other should be 2.

therefore, c.d$ = 1*2 = 2

• hence, the difference = 72-2 = 70

• thus,  the largest possible value of the difference $a.b - c.d$ = 70