Please can any help me with this puzzle????

[Reddevil1976]

I am stuck with this maths puzzle and any help would be very much appreciated. The puzzle is as follows:

Complete the series. 9=4, 21=9, 22=9, 24=10, 8=5, 7=5, 99=10, 100=7, 16=B

I have been trying to solve it but I have got lost with it.

Many thanks in advance.

[bibblebub]

number of letters
9 = NINE - 4 letters
21 = TWENTY-ONE = 9 letters

...
16 = SIXTEEN - 7 letters

[TheNovice]

16 = 7, number of letters

[Fibonacci]

Looks like
NINE - 4 letters
TWENTY ONE - 9 letters
TWENTY TWO-9 letters
TWENTY FOUR - 10 letters
EIGHT - 5 letters
SEVEN - 5 letters
NINETY NINE - 10 letters
HUNDRED - 7 letters
SIXTEEN - 7 letters B=7 ?