focus it's very easy

Hey that's easy indeed 'just focus'

The nth number in the give sequence is 3×(nth prime number)

Or it is defined by the function

f(n)= 3×p_n

where p_n is the nth prime number

So, the missing number = 3×( 5th prime number)

= 3×11

= 33

it's prime number multiplied by 3 so the answer is 33 which is 11*3

They all are in the form 3* prime nos. hence after 7, 11 is the prime no. so solution becomes 3*11 or 33