Best way to convert large fractions to decimals

mental math
New answer on Nov 18, 2019
6 Answers
1.1 k Views
Anonymous A asked on Nov 16, 2019

E.g. I got stuck on how to do 11/275 during a case

What is the best trick for something like this and many others

Are there any recourses to practice large fractions?

Overview of answers

Upvotes
  • Upvotes
  • Date ascending
  • Date descending
Best answer
Francesco
Expert
Content Creator
replied on Nov 18, 2019
#1 Coach for Sessions (4.000+) | 1.400+ 5-Star Reviews | Proven Success (➡ InterviewOffers.com) | Ex BCG | 9Y+ Coaching

Hi Anonymous,

assuming you can approximate numbers, in this case you can find what is a relatively small percentage (eg 1%) and see how the number you have to calculate compares to that.

In this case, 1% of 275 is 2.75, which means 2% is 5.5. At this stage, you would probably notice that 11 is therefore exactly 4%.

Best,

Francesco

Was this answer helpful?
Antonello
Expert
Content Creator
replied on Nov 16, 2019
McKinsey | MBA professor for consulting interviews

Hi,
here you can find a detailed approach to solve it in a quick approximated way and in an accurate way: https://www.youtube.com/watch?v=do_IbHId2Os

I recommend you to take a look at the channel, I really appreciate their video on fast math.

Best,
Antonello

Was this answer helpful?
Deniz
Expert
Content Creator
replied on Nov 17, 2019
BCG | Kearney | 350+ trainees | 5+ years of experience in London, Dubai and Istanbul
Was this answer helpful?
Anonymous replied on Nov 18, 2019

Using percetage may be an easy way to covert complex fractions into decimals.

eg. 11/275

275 = 200 + 75

taking 4% of (200 + 75) = 8 + 3 = 11 thus 11/275 = 4% = 0.04

Other way round if you can image breaking the bigger number

11/275 = 11/5 *1/5*1/11 = 1/25 = 4%

There is no hardcoded rule you may use which ever approach comes to your mind and you seem comfortable with it. Hope this is helpful.

Was this answer helpful?
1
Vlad
Expert
Content Creator
replied on Nov 17, 2019
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School

Hi,

You should approximate it:

50% of 275 = 135

110 is roughly 40%, thus 11 is 4%

Best

Was this answer helpful?
Anonymous B replied on Nov 17, 2019

I typically convert the denominator to 100, so in this case for simplicity I would divide denominator by 3, giving me approximately 3.67%. Then I would round up or down depending on whether I have overestimated or underestimated based on the earlier division, so in this case I would round up and say about 3.9%-4%.

But you should ask if rough estimations are fine. 90% of the time your interviewer will be ok with rounding.

Was this answer helpful?
0
Francesco gave the best answer

Francesco

Content Creator
#1 Coach for Sessions (4.000+) | 1.400+ 5-Star Reviews | Proven Success (➡ InterviewOffers.com) | Ex BCG | 9Y+ Coaching
4,027
Meetings
27,737
Q&A Upvotes
292
Awards
5.0
1480 Reviews