Which one is older three years are 30 months

If you mean 3 years or 30 months then 3 years would be more since there's 36 months in 3 years. :)

3 years since 3 years is 36 months and 30 months is well 30 months. 36 is greater than 30

First we need to write system of 2 equations with 2 variables:
x*6 + y*12.5 = 34
x+y = 3.5

for substitution we express one of variables in one of equations and put it in other equation.
x = 3.5-y

(3.5 - y)*6 + 12.5*y = 34
21 - 6y + 12.5y = 34
6.5y = 13
y = 2
x = 3.5 - y = 3.5 - 2 = 1.5

she ran for 1.5 hours and biked for 2 hours.
check:
1.5*6 + 2*12.5 = 9 + 25 = 34 = 34