Practise the method.Example 1Red : blue counters =2:3. Adding 4 red counters changes the ratio to 3:3. How many blue counters were there?Show worked answerAnswer: 12Write the original counts as 2t and 3t.3t2t+4=33t=4, giving 12 blue counters.Example 25 identical machines finish a job in 15 hours. At the same constant rate, how many machines finish it in 3 hours?Show worked answerAnswer: 25Machine-hours stay constant: 5×15.Divide by 3 hours to get 25 machines.Example 3Red : blue counters =5:6. Adding 3 red counters changes the ratio to 6:6. How many blue counters were there?Show worked answerAnswer: 18Write the original counts as 5t and 6t.6t5t+3=66t=3, giving 18 blue counters.