Creating a feature by combining 2 features with different units?
You can combine multiple features only if they are on same scale. 

Combining feature A with range of possible values (100-1000) with feature B with range of possible values (1-10) will result in feature biased towards A.

You will need to standardize them to the same scale.

For every value X of A = (X - smallest(A))/(biggest(A) - smallest(A)) 
For every value Y of B = (Y - smallest(B))/(biggest(B) - smallest(B)) 

Now all values of A and B resides in the range of (0,1). You can add based on the weight of every attributes. If both are equally important then C = (0.5 * A) + (0.5 * B) and if A should have 70% of weight then C = (0.7 * A) + (0.3 * B)