The cost of a slice of cake is $2.40 x 3/4 = $<<2.4*3/4=1.80>>1.80.
So, 2 slices of cake cost $1.80 x 2 = $<<2*1.80=3.60>>3.60.
Therefore, 2 slices of cake and 1 cup of milk tea cost $3.60 + $2.40 = $6.
#### 6