Question

if 2 = 6, 3 =12, 4 = 20, 5 = 30, 6 = 42 then 9 = ?

Answer 1

At first you may get confused but look at those numbers carefully :)

as you see first result is divisible buy one of the product (second product in all cases is unknown)
so let's check where it will lead us :)

$6/2=3$

$12/3=4$

$20/4=5$

$30/5=6$

$42/6=7$

Now you can clearly see that every number was multiplied by number 1 higher than itself. 
$2*(2+1)=6$

$3*(3+1)=12$

$4*(4+1)=20$

etc...

it means that after equation sign there should be $90$ because 

$9*(9+1)=90$ :)