First find her rate per hour by dividing $90 by 5 hours:
90 / 5 = 18
She makes $18 in one hour.
1 hour = 3600 seconds.
Divide her hourly rate by 3600 to find how much she makes per second:
18 / 3600 = $0.005 per second.
Now divide $0.25 by 0.005 to find the number of seconds needed:
0.25 / 0.005 = 50
It takes 50 seconds.