Please help

We can use complementary counting, and see what are the odds of 0 heads to be up. Since there are 3 coins, and there is a 50% chance to get heads or tails, the odds of having 0 heads is (1/2)^3 = 1 in 8. 

Then, subtracting 1/8 from the total probability (8/8), we get the chance for at least one head to be up as 7/8.

pryap

There are 8 possibilities

   only ONE  ( T T T ) has no 'HEADS'

so  7/ 8 have at leat ONE head

Thanks you so much!