Let the amount of kidney beans to be added be X kg
Cost of X kg kidney bean = $0.89X
Cost of 40 kg pinto bean = $40×0.69 = $27.60
Total amount of the mixture = (40+X)kg
Cost of mixture per kg = $0.81
Total cost of the mixture =$ (40+X)× 0.81
According to question pinto bean and kidney bean are mixed.
Thus,
27.60+0.89X = (40+X)0.81
Removing the parenthesis
27.60+0.89X = 32.4 + 0.81X
Taking like terems together
32.40 - 27.60 = 0.89X - 0.81X
4.80 = 0.08X
X= 4.80/0.08
X = 60kg
Therefore 60kg of kidney beans to be addes to the mixture