Answer
Find out the how many kilograms of blend A will don need to use to make 10 kg of his signature blend .
To proof
As given
Dons signature coffee blend is 60% dark roast and 40% light roast.
He has 10 kg
60 % is written in the decimal form

= 0.6
40 % is written in the decimal form

= 0.4
Now 60% dark roast of 10 kg = 0.6 ×10
= 6 kg
Now 40% of 10 kg = 0.4 × 10
= 4 kg
As given
He has 10 kg of blend A, which is 80% dark roast and 20% light roast i.e 80% of the 6kg is dark roast and 20% of the 4kg is light roast .
80% is written in the decimal form

= 0.8
80% of the 6kg is dark roast = 0.8 × 6
= 4.8 kg
20% is written in the decimal form

= 0.2
20% of the 4kg is light roast = 0.2 × 4
= .8kg
Total kilograms of blend A will don need to use to make 10 kg of his signature blend = 4.8 kg + .8 kg
= 5.6 kg
Hence proved