Answer:
1)  Q ’= 8 Q ,  2)   q ’= 16 q ,  3)  r ’= ¾ r
Explanation:
For this exercise we will use Coulomb's law
   F = k q Q / r²
It asks us to calculate the change of any of the parameters so that the force is always F
Original values
        q, Q, r
Scenario 1
   q ’= 2q
    r ’= 4r
   F = k q ’Q’ / r’²
we substitute
   F = k 2q Q ’/ (4r)²
   F = k 2q Q '/ 16r²
we substitute the value of F
   k q Q / r² = k q Q '/ 8r²
    Q ’= 8 Q
Scenario 2
    Q ’= Q
    r ’= 4r
we substitute
   F = k q ’Q / 16r²
   k q Q / r² = k q’ Q / 16 r²
   q ’= 16 q
Scenario 3
   q ’= 3/2 q
   Q ’= ⅜ Q
we substitute
    k q Q r² = k (3/2 q) (⅜ Q) / r’²
    r’² = 9/16 r²
    r ’= ¾ r