please show me step by step solving this question?

donna buys $2 stamps and $5 stamps. If she spend $54 altogether and get 15 stamps. how many $2 stamps she buy?

ANS:
Let x and y be the no. of $2 and $5 stamps Donna bought respectively.
x+y=15..............(1)
2x+5y=54...........(2)

(1)X5:5x+5y=75......(3)

(3)-(2):3x=21
x=7

the age of a father and son now add up to 40. In six years, the difference in their ages will be 24 years. what are their ages now?

ANS:
Let x and y be the ages of the father and son respectively.
x+y=40......(1)

(x+6)-(y+6)=24
x-y=24..........(2)

(1)+(2):2x=64
x=32

Sub. x=32 into(1),
y=8

1. Let y be the number of $2 stamps she buy.
2y+5(15-y)=54
2y+75-5y=54
-3y=54-75
-3y=-21
y=7
She buys 7 $2 stamps.

2.Let y be the age of the father.
y+(y-24)=40+6
y+y-24=40+6
2y=46+24
2y=70
y=35
The age of the father is 35 and the age of the son 40-35=5 now.