Here's The Solution To That Friends Brain-Teaser

FACT: Joey is older than Chandler (but not by 10 years).

Joey also lies to (younger) Chandler that Monica is younger than Ross.

FACT: Ross is older than Monica.

Ross tells the truth to (older) Monica that he's 9 years older than Rachel.

FACT: Ross is 9 years older than Rachel.

Rachel tells the truth to (older) Monica that she's 7 years older than Chandler.

FACT: Rachel is 7 years older than Chandler.

Chandler tells the truth to (older) Monica that Monica's age is 70% greater than his own. For Monica's age to be a whole number, Chandler's age must be a multiple of 10. Since Monica is older than Ross, and Ross is 7 + 9 = 16 years older than Chandler, that means Monica has to be more than 16 years older than Chandler. The lowest multiple of 7 greater than 16 is 21.

FACT: Chandler is at least 30 years old (and definitely a multiple of 10).

At this point, Monica appears to be the oldest, lying lady. Let's assume that, and see if it works.

In that case, Joey is lying to Ross that the difference in their ages is 6 years, but Monica tells the truth to (older) Joey that the difference between Joey's age and Ross's is the same as the difference between Ross's and Rachel's, namely, 9 years. Let's test this scenario, assuming Chandler's age is 30. Then we get, from youngest to oldest: