Please attempt the above question, timed and then click below to reveal the few second technique leading to the correct answer ==>

Technique © :
Step 1. Total mass of all fifteen children =>

3 ^{1/3} x15 = 50 stones

Step 2. Total mass of ten girls in school year 4 = 50 stones (the answer from step 1) – total mass of five boys in school year 5 (5 x 4 =20 stones) =>

50 – 20 = 30 stones

Step 3. Therefore, the average mass of ten girls in school year 4 is equal to =>

30 / 10 =3 stones

Answer: => Three (3) stones.

Time allowed: 30 seconds

Time taken using the technique: 3 seconds

__Time saved = 27 seconds [9/10__^{th} (90%) of time allowed by examiner]

Note: If you want, you can prove your answer in the following way:

Total mass of boys = 5 x 4 =20 stones

Total mass of girls = 10 x 3 = 30 stones

The total mass of 15 children is 50 (20 + 30) stones

Therefore, the average mass of 15 children is 50 / 15 = 3 1/3 stones. Thus, we have proved it back to the fact given in the question.

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WORK SHEET – Practice question

**Six boys in school year 5 and ten girls in school year 4 have an average mass of three and a half stones. The average mass of ten girls in school year 4 is 3.2 stones. Find the average mass of six boys in school year 5?**

Technique © :

Step 1. =>

Step 2. =>

Step 3. =>

Answer: =>