70 is pronunced "60 10". 71 is "60 11". And so on until 80, which is pronounced, obviously, "4 20". 81 is "4 20 1". It goes up to 90, which is "4 20 10". 91 is "4 20 11". The craziness continues until 100, which is, thankfully, a different word ("cent", instead of "4 20 20", or, dear lord, "60 40"). But in 170 it all comes back.
I think the French-speaking parts of Belgium, Switzerland and Canada did the right thing when they replaced all that complexity by dedicated words for 70, 80 and 90.