Solution: Change the above mathematical problem into mathematical language: 5 yuan +2 yuan +5 corner. Now a * * has been replaced by 30 pieces 1 yuan and 1 corner. How many pieces have been replaced by 1 yuan and 1 corner?

First, convert the units of 5 yuan =50 and 2 yuan =20. Look at the angle first. (Because the number of angles must change more than the number of elements)

Then 50 points were changed to 5 1 minute, 2 yuan to 20 dimes, and now there are 25 dimes, and 5 yuan to 5 ones, which is exactly 25+5 = 30 (pieces). So I changed 25 pieces into 1 corner and 5 pieces into 1 block.

Hope to adopt, I wish you progress in your study! Thank you.