Answer :
let three number as x,y,z
x+y+z=90 —(1)
z=x-5 —(2)
y=3z —(3)
(3) into (1)
x+3z+z=90
x+4z=90
x=90-4z—(4)
(4) into (2)
z=90-4z-5
5z=85
z=17—(5)
(5) into (4)
x=90-4(17)
x=22
(5) into (3)
y=3(17)
y=51
x+y+z=90 —(1)
z=x-5 —(2)
y=3z —(3)
(3) into (1)
x+3z+z=90
x+4z=90
x=90-4z—(4)
(4) into (2)
z=90-4z-5
5z=85
z=17—(5)
(5) into (4)
x=90-4(17)
x=22
(5) into (3)
y=3(17)
y=51