Riko will be behind Yuto until her distance traveled matches his. That is, she will be behind for ...
0.35t < 5.25 +0.25t
0.10t < 5.25
t < 52.5
Riko will be behind Yuto on the interval 0 ≤ t < 52.5 minutes.
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<em>Additional comment</em>
distance = speed × time
Here, time is measured from when Riko starts riding. In addition to the 5.25 miles that Yuto has already gone, his distance will be the product of his speed (0.25 mi/min) and the travel time (t min). Then Yuto's total distance is 5.25+0.25t miles. The speed×time product is also used to find Riko's distance traveled. In her case, it is 0.35t miles.
