

駅まで歩いて三十分です。
先生は三十歳ですか。
今日は三十度です。暑いですね。
"さんじゅう(三十)" is in the lesson "Big numbers" (Level 4), along with 11 other useful words. LinDuo has 180 topic lessons like this one, 2,379 words in total. Every word has an illustration, a transcription and pronunciation in a female and a male voice. Just 10–15 minutes a day is enough to learn them all. Free, and works offline.