If Barbara can walk #3200# meters in 24 minutes:
If we note that #3# minutes is #1/8# of #24# minutes
then we can see that in #3# minutes she can walk
#color(white)("XXX")1/8" of "3200# meters #=400# meters.
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An alternate approach:
We assume that the ratio of distance walked to time is constant (that is that Barbara will walk at the same rate no matter how far or how long she walks).
If we let #x# represent the distance Barbara can walk in #3# minute, then if the distance to time ratio is a constant:
#color(white)("XXX")(x" meters")/(3" minutes")=(3200" meters")/(24" minutes")#
#color(white)("XXX")rArr x" meters" =(3200" meters")/(24" minutes")xx 3" minutes"#
#color(white)("XXXXXXXXXXX")=(3200" meters")/(24 cancel" minutes")xx3cancel(" minutes")#
#color(white)("XXXXXXXXXXX")=(3200" meters")/(cancel(24)_8)xx cancel(3)#
#color(white)("XXXXXXXXXXX")=400" meters"#
