This is how proportion is written 7:21=3:9. The colons are sign of division so it can be shown in the terms of fraction as 7/21=3/9 or 1/3=1/3
√3 - √169 =?
√3 - √(13)² = √³ - 13 or equal √3 + 13 since √169 = + or - 13
800/100 = 8
800 is 100 times as much as 8
