Think of the blend as one whole quantity split between the Rs. 60 coffee and the Rs. 90 coffee. Let \( f \) be the fraction of the total that is the Rs. 60 coffee, so \( (1-f) \) is the fraction that is the Rs. 90 coffee. The blended price is the weighted average of the two prices: \[ 60f + 90(1-f) = 80. \] Expanding, \( 60f + 90 - 90f = 80 \), so \( -30f = -10 \) and \( f = \frac{1}{3} \). This means one third of the total is the Rs. 60 coffee and two thirds is the Rs. 90 coffee, a ratio of \( \frac{1}{3} : \frac{2}{3} \), which simplifies to \( 1:2 \). Compare each option to this fraction split.
Solving for the fraction of cheaper coffee needed in the blend gives exactly \( 1:2 \), matching option (C) alone.
Therefore, the correct answer is 1:2.