three unbiased coins are tossed simultaneously.what is probability of getting atmost two heads?

 Three unbiased coins are totossed. The situation  is as
follows:

No head :      TTT......3C0 = 1 way only  this can
happen

one head:     HTT  or THT or TTH  = 3C1 =  one of
these 3ways the event  can happen

2heads:        HHT 
or HTH or THH = 3C2 =  one of these 3 ways theis event can
happen

All 3 heads: HHH onyl 1 way this
event,

So in a toss  of 3 unbiased coins, the above one of
the 4 types of event can happen in any one of 8 different ways. But getting atmost
2heads is the event of 1st type  or2nd type  or 3rd types  in any one of 1+3+3
= 7 ways.

So probablity getting atmost 2 heads = Pr (0 head
or 1 head or 2heads ) = pr(0 head)+Pr(1head)+pr(2H)  1/8+3/8+3/8  = 7/8
.