"Jeannie is 8 years older than her younger sister April"
this translates to
J = A + 8
"added together, their ages equal 198"
this translates to
A + J = 198
Plug the first equation into this one and we get
A + A + 8 = 198
add like terms
2A + 8 = 198
subtract 8 from both sides
2A = 190
divide both sides by 2
A = 190/2 = 95
Therefore April is 95 years old