Two $20 bills amount to $20 x 2 = $<<20*2=40>>40.
Three $5 bills amount to $5 x 3 = $<<5*3=15>>15.
So, Marj has $40 + $15 + $4.50 = $<<40+15+4.5=59.50>>59.50 in her wallet.
If she buys a cake, she will be left with $59.50 - $17.50 = $<<59.5-17.5=42>>42.
#### 42