If the cost of the 20 pens is Rs.180, calculate the cost of 9 pens

Explanation

This equation is equivalent to the system
`{(2x^(2)+5xgt0),(x^(2)-1gt0),(x^(2)-1!=1),(x^(3)+6=2x^(2)+5x):}`impliesP(xlt-5/2` "and" xgt0),(xlt-1 "and "xgt1),(x!=+-sqrt(2)),(x=-2,1,3):}`
Hence `x_(1)=3` is only root of the original equation.`