let the amount that Alynna save each day be 1u

since Rachel saves $2 more than Alynna,
so Rachel saves (1u+2)

after 10 days,
Alynna has added 10 x 1u = 10u
Rachel has added 10 x (1u+2) = 10u + 20

total savings
Alynna -> 90 + 10u
Rachel -> 10 + (10u+20) = 10u + 30

Alynna has twice as much as Rachel,
so, 90 + 10u = 2 x (10u + 30)
90 + 10u = 20u + 60
gives 1u = 3

Ans: Alynna saves $3 each day

check ...
each day, Alynna saves $3, Rachel saves $5
after 10 days, Alynna added $30, Rachel added $50
total, Alynna -> 90+30 = 120, Rachel -> 10+50 = 60
120 = 2 x 60

Hi, is there a solution that is non-algebraic? P6 only learns simple algebra.