applications of simultaneous linear equations in two unknown

1)In a certain year,the annual simple interest rates for banks A and B are fixed.If Eva desposits $25000and $30000 in banks A and B respectively,she will get a total annual interestof $305.If she deposits$20000 and $35000 in banksA and B respectively,she will get an interest of $155 for half a year.Find the annual simple interest rates for the two banks.
let the interest rates for bank A is x , bank B is y
then
25000x+30000y=305
20000*0.5*x+35000*0.5*y=155
That is
25000x+30000y=305...(1)
10000x+17500y=155...(2)
from (1)
x=(305-30000y)/25000
sub into (2)
10000[(305-30000y)/25000]+17500y=155
122-12000y+17500y=155
5500y=33
y=0.006=0.6%
x=(305-30000y)/25000=0.005=0.5%
the interest rates for bank A is 0.5% , bank B is 0.6%
2) Martin's age is seven times his son's.Ten years later,Martin' ;;s age will be three times his son's.how old are they now?
let Martin's age now is x
then his son is x/7
Ten years later,Martin' ;;s age will be three times his son's
x+10=3[(x/7)+10]
x+10=(3x/7)+30
4x/7=20
x=35
Martin's age now is 35, his son is 5