Answer
28.3
Work Step by Step
$20^2+20^2=x^2$
$400+400=x^2$
$\sqrt {800}=x$
$\sqrt {800}\approx 30$
Divide and find the average to approximate the solution.
$800/div30=26.67$
$(30+26.67)\div2=28.335$
Repeat to further refine.
$800\div28.335=28.219$
$(28.219+28.335)\div2=28.277$
