Mareo is twice as old as his friend Peter. Peter is 5years olderthan Alice. In 5years John will be three times as old as Alice. How old is Peter now?
M=2P
P=5+A
J+5=3(A+5)
So you have J, A, P, M as unknowns, and three statements. You can't solve all with a unique solution.
One of many solutions could be
P=10
A=5
M=20
J=25