DS testing odd & even cases

[Ineedhelp]

I'm reviewing the NP book, 5th edition. I'm on page 78.

The question states

If a,b, and c are integers and ab+c is odd, which of the following must be true?

1. A+C is odd
2. B+ C is odd
3. ABC is even

The question states abc are integers, so -1,2,3 could work to test the possibilities. Is it faster to plug numbers in opposed to memorizing the o-e rules?

[RonPurewal]

Is it faster to plug numbers in opposed to memorizing the o-e rules?

you should be equally comfortable with both approaches. that's the secret to this test: you just want to accumulate as many approaches as possible. don't fall into the trap of trying to seek out the "best" or "fastest" method; that's just going to distract you and/or cause you to hesitate when you already have a perfectly good solution in hand.

so, always review as many approaches as possible when you go back and study the problems (not when you do them under timed conditions!).
for instance, if you solved the problem by testing cases, then go back later and try it with the abstract odd/even rules. if you used the rules, then go back later and try testing cases.

--

also, it's doubtful that you'd have to memorize any rules for odd/even; you've probably known all the relevant "rules" (odd x odd = odd, odd x even = even, etc.) since you were eight or nine years old.
instead, it's more a matter of being able to combine and manipulate those rules with sufficient dexterity, a skill that's best developed by practicing it when you solve and/or review the problems.

[Ineedhelp]

Ron,

Your right, I actually fell for that trap. I started testing numbers, then I started with the odd even approach, then I completely lost myself altogether by not focusing on the task on hand. Thank you, it actually makes a lot of sense, to use and to be comfortable with both methods.

Thanks.

[jlucero]

Glad it helped.