Respuesta :

caylus
Hello,

Let's assume a the age of Bea
b the age of her sister

This year: a+b=18

Next year (a+1)+(b+1)=(a+b)+(1+1)=(a+b)+2=18+2=20

apologiabiology
b=bea's age

sister+b=18
next years, both grew 1
sister+1+b+1=18+1+1=20

sum is 20