Guessing you need to know how old each girl is. Yvonne is 11, her sister is 14.
s = sister
y = Yvonne
Yvonne is 3 years younger than her sister
y = s - 3
s + y = 25
So we’ll substitute the first equation into the second:
s + (s - 3) = 25 and solve for a numerical value for s
2s - 3 = 25
Add 3 to both sides:
2s = 28
s = 14
Yvonne is 3 years younger, so she is 11.
