If M is Mae’s age, then Arna is 2M years old.
In 4 years’ time they’ll be M+4 and 2M+4 year’s old, and we can write: 2M+4=(5/3)(M+4), which means that Arna will be 5/3 times as old as Mae.
Multiply both sides by 3:
6M+12=5M+20,
6M-5M=20-12=8, so M=8, Mae is 8 years old now, so Arna is 16 years old.
(In 4 years Mae will be 12 and Arna 20, 20/12=5/3.)