Let number of copper coins = c

number of bronze coins = b

.20c + .25 b = 2.80

=> 20c + 25b = 280

=> 20c = 280 - 25b

=> c = 14 - (25/20)b = 14 - (5/4)b

To minimize c , we need to maximze b . b needs to be multiple of 4

b = 4 , c = 14 - 5 = 9

b = 8 , c = 14 - 10 = 4

b = 12 , c = 14 - 15 = -1 , Number of coins can be negative .

Therefore , minimum number of copper coins = 4

Answer A

Alternatively ,

we use backsolving here . It will be much quicker .

if b= 0 , 2.8 is not a multiple of .25 . Not possible

b = 1 , 2.6 is not a multiple of .25 . Not possible

b = 2 , 2.4 is not a multiple of .25 . Not possible

b= 3 , 2.2 is not a multiple of .25 . Not possible

b=4 , 2.0 is a multiple of .25 .

Answer A

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