DC Motor starter resistances values.

Hello,
I am reading a french book about electrical engineering (electrotechnique_wildi-sybille), and i am getting difficulties solving the last problem of chapter 28-DC Motor, exercice 29.
Please see the attached PDF file.
The only result i managed to find is R1+R2+R3+R4 = 0.75 Ohms.

The main difficulty for me is :
If i add a resistance in series with the armature, what would be the new speed and armature current ?

Thanks.