Question 737120
The distance from the center of the pipe to the surface
of the water in the pipe is {{{ 25 - 18 = 7 }}} cm
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Also, the distance from the center of the pipe to where
the surface contacts the pipe is the radius, {{{ 25 }}}
This forms a right triangle when you include the surface
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Call ( center of water surface to where surface meets pipe ) = {{{ x }}} 
Now I can say
{{{ 25^2 - 7^2 = x^2 }}}
{{{ 625 - 49 = x^2 }}}
{{{ x^2 = 576 }}}
{{{ x = 24 }}}
The is 1/2 of the width of the water's surface, so
 the width of the water surface across the pipe is 48 cm