T=650mi(hr/50mi)=13 hr
t-1=12 hr
12=650/(50+v) multiply both sides by 50+v
600+12v=650 subtract 600 from both sides
12v=50 divide both sides by 12
v=25/6 mph
v≈4.17mph
Choice D will create an irrational number when added. This is because a rational number plus a rational number will always be irrational (you can experiment this by trying to add rational numbers to pi). For example, 3 + pi = 6.1415926..... or

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:)