Mathematics
posted by James .
A shopkeeper mixed coffee powder worth $2.50 per kg with coffee powder worth $3.50 per kg, and sold 20 kg of the mixture at $2.80 per kg. Find the weights of the 2 grades of coffee powder that he mixed together. Explain clearly your steps of solving it.

Let the amount of the $2.50 coffee be x kg
then the amount of the $3.50 coffee will be 20x kg
2.5x + 3.5(20x) = 2.8(20) = 56
times 10
25x + 35x(20x) = 560
25x + 700  35x = 560
10x = 140
x = 14
He should mix 14 kg of the $2.50 and 6 kr of the $3.50 coffee. 
Weight of $2.50 coffee be x kg
Weight of $3.50 coffee be y kg
2.5x+3.5(20x)= 2.8(20)
2.5x+7003.5x=56
x=5670
x=14
14 kg of $2.50 coffee
2014kg of $3.50 coffee
Thus
14 kg of $2.50 coffee
6kg of $ 3.50 coffee