Let acceleration of 100 kg block =a1
FBD of 100 kg block w.r.t ground

F−T−N1=100a1 ... (i)
FBD of 20 block wrt 100 kg

T−20 g=20(2)
T=240 ... (ii)
N1=20a1 ... (iii)
FBD of 10 kg block wrt 100 kg

10a1−240=10(2)
a1=26 m/s2
F−240−20(26)=100×26
⇒F=3360 N