The minimum input force she'll need to lift the ball is 35 N.
Explanation:
Mechanical advantage of a single pulley is 1. As, she applies 70 N of force to lift the bowling ball, so the output force(weight of the ball) is also 70 N.
Now, adding another pulley gives a mechanical advantage of 2. We have,
M.A = (Output Force)/(Input Force)
Substituting the values we get,
[tex] 2=\frac{70}{F_{i}} [/tex]
[tex] F_{i}=\frac{70}{2} [/tex] = 35 N
Input force equals to 35 N needs to be applied.