You are then left with the most difficult problem of all.
At certain times in their development young people will face particularly difficult problems.
Americans in the past have often put off action on difficult political problems.
"It was a very difficult problem in trying to raise the attention of the right people."
Indeed we seem to have a difficult problem upon our hands.
This is a difficult problem and an active area of medical research.
This is a very difficult problem for the police to deal with.
Particularly when he had a difficult problem like this one.
An easy answer to a difficult problem is at hand.
What looks very simple, is in fact a difficult problem.
Is the difference between hard and easy problems really so important?
And at first it doesn't seem to be such a hard problem.
So it turns out that this is a hard problem, like I said.
The hard problems are often those of control and communication.
It depends on a number of hard problems, first among them population.
But at home he is determined only to avoid the hard problems.
The reason we're here is to solve the hard problems.
That is also a hard problem, though I think progress has been made.
It's a hard problem, but those are always the interesting ones.
These are all hard problems for a computer to solve.