Question 1194310
Let {{{x}}}  lb of cement to be added
given:
{{{ .20*420lb= 84lb }}} of sand initially


{{{ (x + 84) / (x + 420) = .25 }}}
{{{ x + 84 = .25*(x + 420) }}}
{{{ x + 84 = 0.25 x + 105 }}}
{{{ x-.25x = 105-84 }}}
{{{ .75x = 21 }}}
{{{ x = 21/.75 }}}
{{{ x = 28 }}}
{{{28lb}}}  of sand must be added


check:

{{{ (28+ 84) / (28 + 420) = .25 }}}
{{{ 112 / 448 = .25 }}}
{{{ .25 = .25 }}}