In this problem, the value of charge is not mentioned. But the magnitude of electric field is given as 2*10^4 vm^-1. With all the given information we can solve the problem as below. Here I have taken the charge to be 2*10^-8 C (assumed from similar problems).

Given: mass = 80 mg = 80 × 10 ^-6 Kg

Charge q = 2 × 10 ^-8 C

E = 2 × 10 ⁴ Vm ^-1

Let 'T' is tension in string and θ

is angle of string with vertical.

In equilibrium,

Tsinθ=qE ..................(1)

Tcosθ=mg ....,,,,,,,,,,,,,(2)

Divide (1) by (2), we get

tanθ=qE/mg

=> tanθ=[2*(10^−8)*2*(10^4) ]/ [80×(10^−6)×10]

=> tanθ=0.5

=> θ=27

Now Tension T = qE/sinθ = 2×(10^−8)×2×(10^4) / s in27°

T = 8.8 × 10 ^-4 N