Answer:
2 hours; 1/4t=1/2
Step-by-step explanation:
If one half hour is 1/4 of the time they spent at Mount Rushmore, then 4 half hours is the total amount of time they spent there. 4 half hours is the same as 2 hours.
Since 1/4 of the time they spent there was 1/2 hour, the equation 1/4t=1/2 is correct.