Question

of the three numbers the sum of the first two is 45 the sum of the second and third is 55 and the sum of the third and thrice the first is 90 find the third number

Answer 1

a+b=45
b+c=55
c+a=90

c-a=10
c=50
a=40
b=5

Answer 2

let x be the first  number
let y be the second number
let z be the third number
given,
x+y=45
x=45-y............(i)

y+z=55
z=55-y............(ii)

z+3x=90
substituiting the values of z and x from (i) and (ii)
55-y+3(45-y)=90
55-y+135-3y=90
190-4y=90
-4y=90-190
-4y=-100
4y=100
y=100/4
y=25.............(iii)

substituiting (iii) in (i) and (ii)
x=45-y
x=45-25
x=20

z=55-y
z=55-25
z=30

so the third number is 30