First con­sider Linda, a 31-year-old woman, sin­gle and bright. When she was a stu­dent, in high school and in col­lege too, she was deeply involved in social jus­tice issues, and also par­tic­i­pated in envi­ron­men­tal protests.

Which is more prob­a­ble about Linda's occu­pa­tion today?

• a) Linda works as a TV reporter;
• b) Linda is a bank teller;
• c) Linda is a bank teller full-time, but remains very active in the envi­ron­men­tal movement.

Quick, what's your answer?

a) or b) or c)? And, in what pre­cise order?.

Here's the solu­tion:

First, ignore how you ranked a), as it is irrel­e­vant to this mind teaser.

The key is this: If you ranked c) as more prob­a­ble than b), you are wrong…and in very good com­pany.

That is what most peo­ple tend to answer the first time they face this par­tic­u­lar brain teaser, and it reflects a very per­va­sive cog­ni­tive bias, tech­ni­cally called a "con­junc­tion fallacy."

Sta­tis­ti­cally speak­ing, it is more prob­a­ble that Linda is a bank teller, of any kind, that she is both a bank teller full-time and also active in the envi­ron­men­tal move­ment, which is a sub­set of the whole cat­e­gory of all bank tellers.

Makes sense?

And here you are a few more fun and quick brain teasers to exercise your mind. Try to GUESS the answers to the ques­tions below based on your own approach.

The goal is not to find out (or Google) the right answer, but to iden­tify the logical approach that will help "guessti­mate" an appro­pri­ate range, say + or - 30% of the actual answer, and then to com­plete the cal­cu­la­tions (ide­ally men­tally, but you can also take notes) to pro­vide an estimate.

Ready. Set. Go!

Brain teaser questions:

1) How many times heav­ier than a mouse is an elephant?.

2) How many fire­fight­ers are there in San Francisco?.

3) How many trees are there in NYC's Cen­tral Park?

Again, the key here is to exercise your brain and cognitive skills, to plan the steps towards the solu­tion, and then to do the men­tal cal­cu­la­tions to find a rea­son­able range. The goal is not to find the pre­cise cor­rect answer.

Here are the solutions.

1) Around 150,000. An aver­age ele­phant weighs 4,000 kg on aver­age; an aver­age mouse 25 grams.

2) Around 350 fire­fight­ers on duty on any given day, out of a pool of 1,700 fire­fight­ing over­all staff.

3) There are over 26,000 trees (of approx­i­mately 175 species) in the Park.

How close did you come?

--> More brain teasers to help teens and adults test cog­ni­tive skills…and cog­ni­tive biases:

• Test your brain with these 10 visual brain teasers
• Check out SharpBrains' brain twisters and brain games sec­tion, with plenty fun, free mind games for teens, adults and seniors