Greg needs at least $1.60 in stamps to mail a package. He has 28 cents and 4 cent stamps. He can use no more than twenty 4 cent stamps as he only has one book left.

A 5 28cent stamps And 5 4cent stamps

Number of Cents greg requires to mail a Package = $ 1.60

Number of Cents Possessed by Greg = 28 cents

1 Cent = 0.01 $

28 Cent = 28 * 0.01 $=0.28$

Number of cent in form of Stamp that greg has = 4 Cent Stamps

Number of Cent stamps that Greg can use=24 Cent Stamp→$0.24

Amount of money that greg can spend to mail the package

= $0.28+4*$0.24

=0.28+$0.96

=$1.24

So, Greg can't mail this package, if he needs $1.60 to mail the package.