#### Answer

$\approx55.1$

#### Work Step by Step

$66^2+x^2=86^2$
$4356+x^2=7396$
$x^2=7396-4356$
$x=\sqrt {3040}$
$\sqrt {3040}\approx50$
Divide and find the average to find the approximate solution.
$3040\div50=60.8$
$(50+60.8)\div2=55.4$
Repeat to further refine,
$3040\div55.4=54.87$
$(55.4+54.87)2=55.14$