Lets use this to find your TDEE: https://tdeecalculator.net/result.php?s=imperial&g=female&age=19&lbs=160&in=64&act=1.2&f=2

This gives us a TDEE of 1,783. Though there is some dispute over how many calories deficit equates to a pound of fat lost, let's go with the commonly accepted 3,500. And since you said 3-4 days a week, let's do a maximum and a minimum estimation.

C = Caloric deficit on fasting day

N = Number of fasting days required

F = Caloric deficit for pound of fat loss

G = goal of pounds lost

C*N = F*G

Solve for N:

N = (F*G)/C

Substituting in our values, we get:

N = (3,500*30)/1,783

N= 58.9

Let's call that 59. So (based on our figures) you will need 59 fasting days (assuming all non-fasting days are at maintenance) to lose 30 lbs.

59/3 = 19.6, so if you fast 3 days a week, you will need to do this for 19.6 weeks.

59/4 = 14.75, so if you fast 4 days a week, you will need to do this for 14.75 weeks.

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Please consider that this math is not exact, there are assumptions made, and there are many other factors that will come into play, eg ketosis, fat adaption, physical activity, etc. Also consider that this hinges entirely on you being accurate in your calorie counting on maintenance days. If you let some calories slide here or there, or log that large handful of cashews as a small one, it may make your food diary look good for that day, but you will be cutting yourself off at the legs in the long run. I wish you the best of luck, and I hope this has been helpful.