The 14 digits of a credit card are written in the boxes shown. If the sum of any three consecutive digits is 20, what is the value of A?

9

Step-by-step explanation:

Working backwards, the blank box between the 7 and 4 at the end must be 9 in order for the three digits to equal 20.  Then we see the pattern of 947 repeated.  The whole number would be 94794794794794.  That way, any three consecutive numbers would be 20.  The number in the box marked a would be 9.

step-by-step explanation:

for exempl this   system of equations   y = x - 3

                                                                y = 2x - 5

look   the line : y = x -3     ( red)

                            the line : y = 2x -5 ( bleus)