Let the cost of a pencil be "x" and the cost of a biro be "y".
From the first statement, we can write:
2x + 3y = 50 ...(1)
Similarly, from the second statement, we can write:
3x + 4y = 70 ...(2)
We now have two equations with two unknowns. We can solve for x and y using elimination or substitution method. Let's use the elimination method:
Multiplying equation (1) by 3 and subtracting it from equation (2) multiplied by 2, we get:
(2)(3x + 4y) - (3)(2x + 3y) = (70)(2) - (50)(3)
Simplifying, we get:
2x + 5y = 40 ...(3)
Now we have two equations with two unknowns:
2x + 3y = 50 ...(1)
2x + 5y = 40 ...(3)
Subtracting equation (1) from equation (3), we get:
2y = -10
Therefore, y = -5
Substituting y = -5 in equation (1), we get:
2x + 3(-5) = 50
Simplifying, we get:
2x = 65
Therefore, x = 32.5
Hence, the cost of a pencil is 32.5 cents and the cost of a biro is -5 cents.
However, it is not possible for the cost of a biro to be negative, so there must be an error in the calculations or in the problem statement. Please check the numbers and the problem statement again.