Why is my calculation for velocity using time dilation and speed incorrect?

[seto6]

Homework Statement

was a home work problem. I am redoing it for practice
im stuck on part c

this is the hint for part C

Homework Equations

t=T/(1-(v/c)²)^1/2

The Attempt at a Solution

my attempt.
i know that from the restaurant to school it will take 15 min in school's frame and ice cream melts in 5min in the car riders frame..so i did

(15/60)=(5/60)*(1-(v/c)²)^-1/2
so i solved for v and got c=37.74. which was wrong, do not know where I am going wrong.

 

[seto6]

just figured it out

the improper time should be 7.5/v then solving for v should be fine.

 

[seto6]

bump.