Let x be the amount of Ann money, y be Jessica's money and z Mary money.
30% of z equates 0.3*z.
Ann has 30% less than z so: x = z - 0.3z = 0.7z.
70% of z equates 0.7z.
"Jessica has 70% more than Mary " we deduce the equation: y =z+0.7z= 1.7z.
Signe the total is $102, we deduce the equation x+y+z = 102.
x=0.7z and y=1.7z then 0.7z+1.7z+z=102, therefore 3.4z=102 then
z =102/3.4 = $30.
x=0.7*30= 21 and y=1.7*30=51.
Ann has $21, Mary has $30 and Jessica has $51.