In Sam and Jill's garden there are two sorts of ladybirds. There are red Seven-Spot ladybirds with 7 black spots and shiny black Four-Spot ladybirds with 4 red spots.

a red ladybird with 7 black spots and a black ladybird with 4 red spots

Sam and Jill looked at a leaf with three ladybirds on it.

two black ladybirds, one red ladybird

"One Seven-Spot ladybird," said Sam, "and two Four-Spot ones."
"That's 15 spots altogether!" laughed Jill. "I wonder if we could find ladybirds whose spots add to other numbers. I know how to do 16."
"And 14 is easy too," added Sam.

How would you make 16 and 14 spots with the Seven-Spot and Four-Spot ladybirds?

What other numbers under 30 can you make with 4s and 7s?
Which ones are impossible?