Lashonda purchased a prepaid phone for $20 . long distance calls cost 21 cents a minute using this card .lashonda used her card only once to make a long distance call . if the remaining credit on her card is $9.92 , how many minutes did her call last ?

For this case, the first thing we must do is define a variable.
 We have then:
 x: number of minutes
 y: total cost of the call.
 We now write the equation that models the problem:
 
 Since the remaining credit is 9.92 $, then we evaluate the value of y = 9.92 in the equation:
 
 From here, we clear the value of x:
 
 
 The last call was 48 minutes.