Oligonucleotide-based technologies for biosensing or bio-regulation produce huge amounts of rich high-dimensional information. There is a consequent need for flexible means to combine diverse pieces of such information to form useful derivative outputs, and to display those immediately. Here we demonstrate this capability in a DNA-based system that takes two input numbers, represented in DNA strands, and returns the result of their multiplication, writing this as a number in a display. Unlike a conventional calculator, this system operates by selecting the result from a library of solutions rather than through logic operations. The multiplicative example demonstrated here illustrates a much more general capability--to generate a unique output for any distinct pair of DNA inputs. The system thereby functions as a lookup table and could be a key component in future, more powerful data-processing systems for diagnostics and sensing.