This bosom friend is familiar with shopping. For example, a 10% discount on a commodity is 90% of the original price multiplied by 0.9, while the original 100 yuan is only 90 yuan. 25% off is 75% of the original price 100 times 0.75=75 yuan.

And now people have a lot of bank loans, (loan interest rate+1)* loan amount = monthly payment * repayment time. This is a mathematical example.

Another example is someone building a house, and through mathematics, we can find the way to save the most materials and the largest area.