Rohan has a stationery collection, consisting of rulers, pens and erasers. His pencil box has a total of 20 pieces. Rohan as two and half times as many rulers as pens, and the number of erasers is 4 less than the number of rulers. How many erasers does Rohan have?

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Assume that there are R rulers, P pens and E erasers. It is given that: R = (5/2) * P or P = (2*R)/5 And, the number of pairs of erasers is 4 less than the number of rulers. E = R – 4 or R = E + 4 Also, combining the above, we have P= {2*(E+4)}/5 Also, there are total 20 pieces. R + P + E = 20 {2*(E+4)}/5+E+4+E= 20 Multiply both sides by 5 and carrying out the necessary operations, we have 12E+28= 100 E = 6 Therefore, Rohan has a total of 6 erasers.

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