Test scores needed for a target mean

Ben scored \(68\), \(74\) and \(81\) marks in three tests. Every test is marked out of \(100\).

1. Find the least score he needs in the fourth test for his mean over the four tests to be at least \(75\).[2]
2. After four tests his mean is exactly \(75\). Show that his mean over five tests cannot be more than \(80\).[2]
3. He scores \(x\) marks in the fifth test. Find the range of values of \(x\) for which his mean over the five tests is from \(76\) to \(79\) inclusive.[2]