The variables are:
x: quantity of Queen City coffee blend, in pounds
y: quantity of Arabian Mocha coffee blend, in pounds
10 pounds of a coffee are needed, then:
x + y = 10 (eq. 1)
Queen City coffee is sold for $11.90 per pound, Arabian Mocha coffee is sold for $13.10 per pound, the blend must be sold for $12.86 per pound, then:
11.9x + 13.1y = 12.86*10
11.9x + 13.1y = 128.6 (eq. 2)
Isolating x from the first equation:
x = 10 - y
Substituting the last equation into the second equation:
11.9(10 - y) + 13.1y = 128.6
11.9*10 - 11.9y + 13.1y = 128.6
119 + 1.2y = 128.6
1.2y = 128.6 - 119
1.2y = 9.6
y = 9.6/1.2
y = 8
And the value of x is:
x = 10 - 8
x = 2
They must mix 2 pounds of the Queen City Blend and 8 pounds of the Arabian Mocha Blend.