That formula doesn't have a name since I made it up from the data you provided, but here is the derivation. Call it the A2K formula if you must.
The fraction of mall goers in the total population is n1/N.
The fraction of 18-65 year olds in the total population is n2/N
The fraction of cell phone users is n3/N.
The fraction of all three is the product of those three numbers so n1*n2*n3/N^3 assuming they are independent.
The number of people total is that fraction times the total population or n1*n2*n3/N^2