Question

last year my sister was 7/8 the age of my brother, how old is my sister?
age difference a year ago is b= s+1/8 age ratio s = 7/8 b
therefore s = 7/8 (s+1/8)

is this correct logic?

Answer 1

let brothers age last year be x
then sister's age last year = 7x/8

brother's age now = x+1
sister's age now = 7x/8 + 1 = (7x+8)/8

we need one more piece of information to actually find the age.

e.g. if brother last year was 8
then sister last year was 7
now brother is 9 and sister is 8

if brother last year was 16
then sister last year was (7/8)(16) = 14
brother now is 17 and sister is 15
etc.