Set up and solve a system of equations.
Let x represent the cost of 1st class stamps
Let y represent the cost of post card stamps
17.10 = 20x + 10y .... (1)
70.00 = 80x + 50y .... (2)
subtracting 4 of equation 1 from equation 2 you obtain the following:
1.6 = 10y
which gives y = .16
Therefore post card stamps cost $0.16
Plugging this back into either equation you get x = .775
Therefore the cost of 1st class stamps is $0.775