benefactor.js

// The accounts of the "Fat to Fit Club (FFC)" association are supervised by John as a volunteered accountant. The association is funded through financial donations from generous benefactors. John has a list of the first n donations: [14, 30, 5, 7, 9, 11, 15] He wants to know how much the next benefactor should give to the association so that the average of the first n + 1 donations should reach an average of 30. After doing the math he found 149. He thinks that he could have made a mistake.

// if dons = [14, 30, 5, 7, 9, 11, 15] then new_avg(dons, 30) --> 149

// Could you help him?

function newAvg(arr, newavg){
    const x = newavg * (arr.length + 1) - arr.reduce((a,b) => a + b,0)
    if(x <= 0) throw "Expected New Average is too low"
    return Math.ceil(x)
  }