Dad said that when you grow to 15, I will be 45.

It means that Dad is 45- 15=30 years old.

Duoduo is 8 years old this year.

Explain that dad is 30+8=38 years old this year.

Method 2:

Flowering to growth is 15 years later.

Seven years later, my father was 45 years old.

Explain that dad is 45-7=38 years old this year.