Yang has a great explanation, but I figured to show an alternate way if you're curious.
She can mow 3/8 of an acre in 45 minutes. So we can effectively write this ratio
3/8 acre: 45 minutes
Divide both sides by 45 so that "45 minutes" turns into "1 minute". Dividing by 45 is the same as multiplying by (1/45) so I'll use this fraction because of the 3/8 fraction
So we have
(1/45)*(3/8) acre: (1/45)*45 minutes
1/120 acre: 1 minute
meaning that Sasha can mow 1/120 of an acre in 1 minute
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The last step is to multiply both sides of this ratio by 60 so that we turn that "1 minute" into "60 minutes". Afterward we replace "60 minutes" with "1 hour"
1/120 acre: 1 minute
60*(1/120) acre: 60*1 minute
60/120 acre: 60 minutes
1/2 acre: 1 hour
So this is another way to show that Sasha can mow 1/2 an acre in one hour