Q. A plant grows by one sixth of its height every 31 days. On 1st of August its height was 21 inches. What was its height on 1st of July of the same year?

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

Hours:Minutes:Seconds
00:00:00 Technique © :

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

Step 2.   Therefore 1/7th 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 1st 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 1st July and calculate 1/6th to arrive at the height of 21 inches on 1st August  18 x 1/6 = 3 inches. Therefore the height on 1st August is 18 + 3 = 21 inches

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

Time saved = 27 seconds [9/10th(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 1/5th 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