c$)$ In a ICP$/$MS experiment a standard solution containing $90ppb$ analyte and $60ppb$ internal standard gave peak areas of $400$ and $350$ for the analyte and the internal standard, respectively. $10c{m}^3$ of the unknown analyte and $10c{m}^3$ of $15ppb$ internal standard were made up to $25c{m}^3.$ When a portion of this solution was run under the same conditions as before, a peak area of $550$ was found for the analyte and a peak area of $600$ was found for the internal standard. Calculate the concentration of the unknown analyte.