Hello,

let x be the number of apple-trees

let y be the number of peach-trees

let z be the number of pear-trees

Then you get a system of equations:

(A) x + y + z = 144

(B) x = y + z

(C) y = 2z

Use substitution:

(C) in (B):

x = 2z + z = 3z

(A) becomes:

3z + 2z + z = 144

z = 24 pear-tress

y = 48 peach-tress

x = 72 apple-trees