Question 1202878
10 years ago, when Su Lin celebrated her birthday with her family, the ratio of the age of Su Lin to Su Lin's sister was 5 : 7. This year, the ratio of the age of Su Lin to Su Lin's sister is 5 : 6. How old is Su Lin now?

 let the common multiple be x

10years ago Su lin was 5x years

Her sister was 7x years

after 10 years 

Su lin is  5x +10 years

Her sister is 7x +10 years

(5x+10)/(7x+10) = 5/6    ( given condition)

6(5x+10)= 5(7x+10)

30x +60 = 35x + 50

35x-30x=60-50

5x=10
x= 2 

Su lin was 2*5 =10  years old 10years ago

Now she is 10+10 = 20 years