If (x × 10^q) – (y × 10^r) = 10^r, where q, r, x, and y are positive integers and q > r, then what is the units digit of y?	Choose one.
A) 0	B) 1	C) 5	D) 7	E) 9
Group the 10^r terms on one side of the equal sign.   (x × 10^q) – (y × 10^r) = 10^r   x × 10^q = (y × 10^r) + (1 × 10^r)   x × 10^q = (y + 1) × 10^r   Now, solve for y.   x∗10 q=(y+1)∗10 rx∗10 q=(y+1)∗10r   x∗10q10r=y+1x∗10q10r=y+1   x∗10 q−r=y+1x∗10 q−r=y+1   x∗10q−r−1=yx∗10 q−r−1=y   Since q &gt; r, the exponent on 10^(q-r) is positive. Since x is a positive integer, x*10^(q-r) is a multiple of 10 and therefore ends 0. Any multiple of 10 minus 1 yields an integer with a units digit of 9.   The correct answer is 9.
<result>E</result>
If (x × 10^q) – (y × 10^r) = 10^r, where q, r, x, and y are positive integers and q > r, then what is the units digit of y?