Value of Exponent

What is the value of $x$ if $4^{20}+4^{20}=2^x$?
Source: NCTM Mathematics Teacher, January 2006

SOLUTION
$4^{20}+4^{20}=2^x$
$4^{20}(1+1)=2^x$
$4^{20}(2)=2^x$
$(2^2)^{20}(2)=2^x$
$2^{40}(2)=2^x$
$2^{41}=2^x$
$x=41$

Answer: $41$