Abbie decides to start doing crunches as part of her daily workout. she decides to do 15 crunches the first day and then increase the number 4 each day after that. write an explicit formula for the number of crunches abbie will do on day n.

For this case, the first thing we must do is define a variable.
 We have then:
 n: number of days.
 We now write the explicit formula that represents the problem.
 We have then:
 an = 4n + 15
 Where,
 15: crunches the first day
 4: increase the number 4 each day
 
 An explicit formula for the number of crunches Abbie will do on day n is:
 an = 4n + 15