Total no. of outcomes = 4  
HH, HT, TH, TT (where H is head and T is tail)

No. of favourable outcomes = 1
HH (coming up two heads together)

Probability = No. of favourable outcomes / Total no. of outcomes = 1/4
 

Answer = 1/3

Let S be the event of tossing two coins

Sample space S = {HH,TT,HT,TH}

Let A be the event where at least one head shows up

A = {HH,TH,HT}

Let  B be the event where two heads show up

B = {HH}

Now,

Probability of event B given that event A happens = P(B/A) = n(B)/n(A) = 1/3