Add spaces: Onceyou'vestarted,youwon'twanttostop.

Answer: Once you've started, you won't want to stop.

Add spaces: Radiographicstudiescanbehelpfulinconfirmingthediagnosis,buteventhistypeofdiagnosticmethodischallenging.

Answer: Radiographic studies can be helpful in confirming the diagnosis, but even this type of diagnostic method is challenging.

Add spaces: Youcanfindtheprobabilityofgetting'heads'inonetossand'tails'intheotherbyaddingtheprobabilities:

Answer:
You can find the probability of getting 'heads' in one toss and 'tails' in the other by adding the probabilities: