Çözüldü Speed, Time, and Distance (8th Grade)

From the University of Colorado - Boulder (I have changed the tenses to have better time consistency):

The Walker family decided to attend a wedding out of town. Betty and Harold Walker left a day early and arrived in 3.5 hours at the town where the wedding was going to take place. If Mary and Jane Walker left the morning of the wedding, drove an average of 10 miles per hour faster than their siblings had done the day before, and arrived in 3 hours, how far away was the town from the Walker Residence?

The average speed of Betty and Harold: v miles / hour
Time for Betty and Harold to arrive: 3.5 hours
The average speed of Mary and Jane: v + 10 miles / hour
Time for Mary and Jane Walker to arrive: 3 hours
Distance from the town to the Walker Residence: D
D = 3.5·v = 3·(v + 10) ⇒ v = 30 / 0.5 = 60 miles / hour
D = 3.5·60 = 210 miles.

Original Question:
https://wetalldid.wordpress.com/wp-...est-1-review-2011-precalculus-carrie-muir.pdf
(page 3, Question 13)
Answers: https://wetalldid.wordpress.com/wp-...ew-solutions-2011-precalculus-carrie-muir.pdf