40 is what percent of 400

x/100 = 40/400

Cross multiply

400x = 4000

Divide by 400 on both sides

x = 100

x = 10%

Since 40/400 is 1/10, and 1/10 is equal to 10/100, which is, again, equal to 10%, that means 40 is 10% of 400.

Good job, saseflower! Good use of proportions and cross-multiplying.