For the sake of showing work I will replace the empty space with an x like so...
![(9)/(10) =(90)/(x)](https://img.qammunity.org/2020/formulas/mathematics/middle-school/a566j4z7cq484vybg4qe7wpqat8dmc2ruh.png)
To find out what x is you must cross multiply (aka butterfly)
***Image of this step is attached below
9*x = 10*90
9x = 900
Next divide 9 to both sides to finish isolating x. Since 9 is being multiplied by x, division (the opposite of multiplication) will cancel 9 out (in this case it will make 9 one) from the left side and bring it over to the right side.
9x ÷ 9 = 900 ÷ 9
1x = 100
x = 100
To make these fractions equivalent the empty spot must be 100
![(9)/(10) = (90)/(100)](https://img.qammunity.org/2020/formulas/mathematics/middle-school/rpgn5yj51sjm9hhs0cmvf4dxjck11o39hn.png)
Hope this helped!
~Just a girl in love with Shawn Mendes