Hannah has a biased coin.
she is going to throw the coin once.
the probability of getting heads is 0.7
work out the probability of getting tails.

Look at it this way:

When you flip a coin, the probability of it landing with EITHER side showing
is 100%. 

This leads us to the rule ...

               The sum of the probabilities of
               all possible outcomes is 100%.

For a coin:  (probability of heads) plus (probability of tails) = 100%.

That just says:  We're 100% sure that the coin will land with either
heads or tails up.

An "honest" coin gets heads 50% of the time and tails the other 50%.

But if the coin is all bent and squashed and has a feather stuck to
one side and a wad of gum on the other side so that it comes up
heads 70% of the time, then the coin isn't 'honest'.  But it still has to
land EITHER heads OR tails, so the sum of the probabilities is still 100%.

So the probability of heads is 30%.