4. A*B means multiply A by B; A@B means divide A by B, A?B means add B to A and A=B means subtract B from A. Then find the value of 10*10 = 5*10 ? 50@10
5. Some equations have been solved on the basis of certain system. Find the correct answer for the unsolved equation on that basis. If 98-39-27=31, 87-38-34=20, then 79-25-12 = ?
6. In a certain code language, '+' represents 'x', '-' represents '+', 'x' represents '÷' and '÷' represents '-'. What is the answer to the following question? 56 ÷ 8 + 12 - 72 = ?