Final answer:
The fraction of tosses that are 'tails' is 4/7.
Step-by-step explanation:
To find the fraction of tosses that are 'tails', you need to subtract the fraction of tosses that are 'heads' from 1. In this case, Martha got 3 'heads' out of 7 tosses. So, the fraction of tosses that are 'heads' is 3/7. Therefore, the fraction of tosses that are 'tails' is 1 - 3/7 = 4/7.