A knife is three times the cost of a spoon. 9 spoons and 12 knifes cost?82.80 workout the cost of 1 knife.

Let S = spoon

Let K = knife

1 knife is thee times the cost of a spoon, so it would be 3S

Now you have 9S + 12K = 82.80

We can replace the K with 3S:

9S + 12 (3S) = 82.80

Simplify the left side:

9S + 36S = 82.80

Add:

45S = 82.80

Divide both sides by 45"

S = 82.80 / 45

S = 1.84

One spoon cost $1.84

We know one knife cost 3 times as much, so multiply the cost of a spoon by 3:

Knife = 1.84 x 3 = $5.52