To be effective, percutaneous transluminal coronary angioplasty (ptca) must be performed within what time frame, beginning with arrival at the emergency department after diagnosis of myocardial infarction (mi)?

This procedure needs be performed within 60 minutes time frame.
This commonly known as "door-to-balloon. 
Granting entry to  the blood in order to circulate unobstructed to the heart muscle, the PTCA is less invasive procedure to open up blocked coronary arteries.

The ratio would be 3:1

2 dozen= 24 so the ratio would be 24:8 but both numbers are divisible by 8