According to the wobble hypothesis, a G at the 5' end of the tRNA anticodon can pair with either a U or a C at the 3' end of a codon. Similarly, a U at the end of an anticodon can pair with either an A or a G, and an I (which is the symbol for the nucleotide base inosine) can pair with A, U, or C. What is the minimum number of different tRNAs that must exist in order for a cell to be able to translate all 61 sense codons?