Find the 25th even integer.

You could write the first twenty five of them and see.
Or you could count them on your fingers to 25, but 
that's probably not what your teacher wanted you to 
do.  I'm guessing you are studying sequences.

2,4,6...

is an arithmetic sequence, the nth term, an, is 
given by the formula

an = a1 + (n-1)d 

where a1 = the first term and n = the number of term.

Here n = 25, so we want to find a25.  The common 
difference between even integers is 2, so d = 2.  
The first term is 2, so a1 = 2.

 an = a1 + (n-1)d
a25 = 2 + (25-1)(2)
a25 = 2 + (24)(2)
a25 = 2 + 48
a25 = 50

I'll bet you could have guessed it would be 
twice 25, couldn't you?

Edwin