Technique © :
Step 1. Realise the fact that if the plant is growing at 1/6^{th} every 31 days, on 1^{st} August its height is 1/7^{th }(increase the denominator by one) the height than 31 days back on 1^{st} July.

Step 2. Therefore 1/7^{th} of 21 inches is 3 inches.

Step 3. Now subtract the answer in step 2 from 21 inches

21 – 3 =18 inches

Answer: The height of the plant on 1^{st} July was 18 inches.

*Note: If you want to mentally double check your answer is right, you can now work forward, starting at the height of 18 inches on 1*^{st} July and calculate 1/6^{th} to arrive at the height of 21 inches on 1^{st} August 18 x 1/6 = 3 inches. Therefore the height on 1^{st} August is 18 + 3 = 21 inches

Time allowed: 30 seconds Time taken using the technique: 3 seconds

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

Now that you have mastered how to answer the above question in only three seconds (90% of the time allowed), go ahead and try a similar question below:

__WORK SHEET – Practice question__

A tree by 1/5^{th} of its previous height every fortnight (two weeks) and its height today is 30 cm. Find the height of the tree a fortnight ago from today.

STEP 1

STEP 2

STEP3