Tell you in detail! There are three unknowns in your equation, and it takes three equations to get the solution of each unknown. So it is impossible to solve every unknown. But all you need now is the sum of x+y+z, and you can do it with your brain. The solution is as follows: subtract one from the second formula to get x+3y= 105, and then change this formula to 2 to get 2x+6y=2 10.

Finally, subtract the formula from the formula just obtained: x+y+z= 105.