20% of 900 is 180. This can be calculated by multiplying 20/100 with 900, which gives (20/100)900 = (20900)/100 = 180. Alternatively, we can use the following steps:

1. Represent 20% of 900 as a fractional multiple: 20% x 900
2. Simplify the expression by dividing 20 by 100: (20/100) x 900
3. Multiply the resulting fraction with 900: (20/100) x 900 = 180

Therefore, 20% of 900 is 180.