tolong ya nomor 11 buatmodelnya dan caranya terima kasih

Mono = 7thn + Edi.... pers. 1
Cahyo = 4thn + Mono.... pers. 2
Mono + Edi = Cahyo.... pers. 3


Edi = Mono - 7thn.... pers. 1

Substitusi pers. 1 dan pers. 2 ke pers. 3

Mono + Edi = Cahyo
Mono + (Mono - 7thn) = 4thn + Mono
2 × Mono - 7thn = 4 thn + Mono
2 × Mono - Mono = 4thn + 7thn
Mono = 11thn