I'm not too sure about this but unit is definitely one of the answers. If "number" refers to (for example) the "6" in 6 inches then it should also count. I think "symbol" could also just be unit so I'm not sure about that.
The butcher will balance the meat and the 0.5 kg mass on one side of the beam balance and 2 kg mass on the other side of the beam balance.
By so doing, when the beam balances then the amount of meat measured will be exactly 1.5 kg as required. Since the 1.5 kg meat and 0.5 kg mass is equivalent to the 2 kg on the other side of the beam balance.
