So if she makes 8 and hour and works 40 hours a week Thats 320 take home every week Now divide that number by 8 as she is only taking home 1/8th and you are left with only 40 dollars saved a week. Meaning to get to 457 it would take 11.425 weeks of work
ALTERNATIVE ROUTE: You know that 0.125 is equal to 1/8 And 1/2^3 is also equal to 1/8 So since you need the reciprocal to match it the exponent is a negative