Question 1140820
x, Son
y, Father

Description literally gives system {{{system(y=2x+15,x+y=40)}}}.


Solve by substitution for y, if you like, gives results, with fractional parts of year for each:

{{{system(x=8&1/3,y=31&2/3)}}}
The boy is 8 years and about 4 months.