John's laptop won't recharge and he suspects the ac adapter is the problem. which tool should he use to test it?

A multimeter

Explanation:

A multimeter -

It is an instrument  which is capable to measure the resistance , current and voltage , is referred to as a multimeter.

It is in a form of a hand held instrument , which can be very conveniently used to check for  resistance , current and voltage .

Hence, from the given information of the question,

The tool which can be used by John is a multimeter.

Explanation:

The following code is written in Java and modified to do as requested. It asks the user to enter a number that is saved to the amountToChange and then uses the % operator to calculate the number of 5 dollar bills and the number of 1 dollar bills to give back. A test case has been provided and the output can be seen in the attached image below.

import java.util.Scanner;

class ComputingChange {

   public static void main(String[] args) {

       Scanner scnr = new Scanner(System.in);

       int amountToChange;

       int numFives;

       int numOnes;

       amountToChange = scnr.nextInt();

       numFives = amountToChange / 5;

       /* Your solution goes here */

       numOnes = amountToChange % 5;

       System.out.print("numFives: ");

       System.out.println(numFives);

       System.out.print("numOnes: ");

       System.out.println(numOnes);

   }

}