The groceries cost $12 x 2 = $<<12*2=24>>24
Including the toilet paper, Monica and Sheila had already spent $12 + $24 = $<<12+24=36>>36
This means that they had $50 - $36 = $<<50-36=14>>14 left to spend on themselves.
However, the boots cost $14 x 3 = $<<14*3=42>>42 per pair.
Two pairs of boots would cost $42 x 2 = $<<42*2=84>>84
They need another $84 - $14 = $<<84-14=70>>70 to buy two pairs.
Each of them needs to add another $70/2 = $<<70/2=35>>35
#### 35