Lily is twelve years old and her brother, Percy, is eight. How old will Percy be when the sum of their ages is forty?
Let x be how much they have aged when the sum of their ages is 40.
12+x+8+x=40
20+2x=40
2x=20
x=10
Therefore Percy will be 8+10=18 years old when the sum of their ages is 40.
