Robert purchased $2,000 worth of U.S. savings bonds. If bonds are sold in $50 or $100 denominations only, which of the following CANNOT be the number of U.S. savings bonds that Robert purchased?

50x+100y=2000
x+2y=40
neither can be 29
neither can be 27
if x is 30 then y is 5
if x is 40 then he only bought $50 bonds
neither can be 50
dang! im confused now O.o

if it's 40, then he bought only $50 bonds, hence $50*40=$2000
if it's 30, then he bought 10 $100 bonds, and 20 $50 bonds,
hence 10*$100 +20*$50=$1000+$1000=$2000
if it's 50, it's not possible, cos no matter the ratio, he will buy more than $2000 worth of bonds
i don't know how to go about 27 and 29