Donald is 6 years older than Sophia. In 4 years the sum of their ages will be 74. How old is Donald now?

Let $D$ denote Dona;d's age and $S$ denote Sophia's age. Then
\[(1)       D=S+6.\]
In four years Donold will be $D+4$ and Sophia will be $S+4$. Hence
\[(D+4)+(S+4)=74   \Rightarrow  D+S+8=74  \Rightarrow \]
\[ (2)     D+S=66.\]
Substituting (1) in (2) we get
\[(S+6)+S=66  \Rightarrow 2S+6=66  \Rightarrow 2S=60  \Rightarrow S=\frac{60}{2}=30.\]
Subsitituting $S=30$ in (1) we get
\[D=30+6=36.\]
So Donold is $36$ years.