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19.      12² + b²  = 20²

20.     5²  +  10²   =   25   + 100  = 125 = c²           ( 'c' then = sqrt (125)  )

ok,

According to the Pythagorean Thereom,

$12^2+b^2=20^2$

$144+b^2=400$

$b^2=256$

$b=16$

This is for 19.

 $25+100=125$

$\sqrt125=15$

so 15 for number 20

waitwaitwait.

$\sqrt125$

$5\sqrt5$

oops. um...