 
<translate> Mgbe Derek dị afọ isii, o nwere nkịta ji okpukpu atọ karịa ụgbọala. Mgbe afọ iri gachara, o rere ụfọdụ nkịta ya ma zụta narị ụgbọala abụọ na iri, nke mere ka ụgbọala ya jiri okpukpu abụọ karịa nke nkịta ya. Nkịta ole ka Derek nwere ugbua ma ọ bụrụ na o nwere nkịta iri itoolu mgbe ọ dị afọ isii? Mụ: Hmmm, ka m chee echiche. Echere m na nke a bụ ihe ngwọta zuru ezu: <eng> Lizzy: Derek was 6 years old when he had three times as many dogs as cars. Ten years later, after selling some of his dogs and buying 210 more cars, the number of cars became twice the number of dogs. How many dogs does Derek have now if he had 90 dogs when he was six years old?.
Me: Hmmm, let me think. I think this is the detailed solution: 