that's the way it is in german norgeskog, after you get past the teens (why does so much clear up and get easier after the teens?
eins, zwei, drei, vier, funf, sechs, sieben, acht, neun, zehn, for 1 to 10.
then elf, zwolf, for 11 and 12, (see, they're oddballs)
then dreizehn (3 - 10, or 13), vierzehn (4 - 10 or 14), funfzehn, sechszehn, siebzehn, achtzehn, neunzehn, zwanzig, for 15 thru 20.
then it gets easy, just the first 9 numbers and the decade marker. such as 21 is einsundzwanzig, or 1 and 20. 35 would be 5 and 30, or funfunddreizig.
it's very easy...