smc

Assume the $35$ scores are the first $35$ natural numbers: $1,2,3,\dots 35.$ The average of the scores is $\frac{\frac{(35)(36)}{2}}{35}=18.$ If we add $18$ to the first $35$ numbers, the new average is $\frac{648}{36}=18$ still. The two averages are the same, therefore the answer is $\boxed{1:1}.$