If you would like to know what percent of the total number of songs has Lani learned, you can calculate this using the following steps:
x% of 84 songs is 21 songs
x% * 84 = 21
x/100 * 84 = 21 /*100
x * 84 = 21 * 100 /84
x = 21 * 100 / 84
x = 25%
Result: Lani has learned 25% of 84 songs in her piano playbook.