6 plates and 5 cups cost 23.80 dollars. 7 plates and 6 cups cost 28.00. Each plate costs the same as the other plates, each cup costs the same as the other cups. What is the cost of 1 plate? What is the cost of 1 cup?

Cups = c
Plates = p

6p + 5c = 23.80 ==> multiply by 6 ==> 36p + 30c = 142.80
7p + 6c = 28.00 ==> multiply by 5 ==> 35p + 30c = 140.00

Now use algebra to have the 30c be on one side and the rest on the other:

36p + 30c = 142.80 | -36p

30c = 142.80 - 36p

35p + 30c = 140.00 | -35p

30c = 140.00 - 35p

Now set them equal to each other about the 30c:

142.80 - 36p = 140.00 - 35p

Use algebra to solve for p:

142.80 - 36p = 140.00 - 35p | +36p

142.80 = 140.00 + p | - 140

2.80 = p

Go back to one of the "30c" equations and plug in the value for p:

30c = 140 - 35p

30c = 140 - 35(2.80)

30c = 140 - 98

30c = 42 | /30

c = 42/30

c = 1.4

Plates are $2.80
Cups are $1.40