Explanation:
x = pounds A
y = pounds B
x + y = 123
4.45x + 5.85y = $643.95
from the first equation we get
x = 123 - y
and we use that in the second equation :
4.45×(123 - y) + 5.85y = 643.95
547.35 - 4.45y + 5.85y = 643.95
1.4y = 96.6
y = 69
x = 123 - y = 123 - 69 = 54
so, he used 69 pounds of B coffee.