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Three small pumps and one additional pump are filling a tank. Each of the three small pumps works at $2/3$ rd the rate of the large pump. If all $4$ pumps work at the same time, then they should fill the tank in what function of time that it would have taken the large pump alone?

1. $4/7$
2. $1/3$
3. $2/3$
4. $3/4$

1