Given 220 volts 15 amperes 700 rpm and 0.8 ohms what speed will it run if a resistance is connected in series with supply voltage assume the motor is taking the same current?

Most probably, it won't even run or it will run for a while and then burn out, a motor that run 700rpm is an 8 pole induction motor, the only way to get it to run slower or faster is to change the supply frequency, say you want it to run at 350 rpm you have to change the supply frequency from 50Hz to 25Hz