Use substitution method.
Let the cost of one pencil, eraser and sharpner be p, q and r. We have the equations,
\\(2p+5q+8r=47\\) and \\(3p+3q+7r=42\\) Multiply Eq. (2) by 2 and subtract Eq. (1) from Eq. (2), we have
\\(4p+q+6r=37\\) multiply this by 3 to get the answer.
Use substitution method for (x-y) & (x+y).
Try to make the coefficients of any one variable equal,
m & n are constants.
Let the boat's speed upstream be x kmph and that downstream be y kmph. Then, distance covered upstream in 8 hrs = Distance covered downstream in 4 hrs.
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