Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

You are not logged in.

- Topics: Active | Unanswered

**Busby****Member**- Registered: 2006-03-03
- Posts: 1

Having problems thinking of what the formula is for calculating the following:-

Say you you have the following:-

Gross income 2005= £8000

If this is a rise of 30% of the previous year then how would you calculate what the previous years gross was?

Same applies if it was 35% , 45% etc.

Thanks

Busby

Offline

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,569

A rise of 30% is another way of saying it was 1.3 times larger (ie 1 + 30/100).

So the previous year was 8000/1.3 = 6153.84.

(6153.84 plus 30% = 6153.84 + 30% of 6153.84 = 6153.84 + 6153.84×0.3 = 6153.84 + 1846.15 = 7999.99)

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

Offline

**John E. Franklin****Member**- Registered: 2005-08-29
- Posts: 3,588

If something goes down by 44% then it is 1 - 44% of 1 or 1 - .44 x 1 or 1 - .44 or

0.56 of what you started with.

If something starts at 56 and ends at 100, then it is a 44% decrease going BACKWARDS in time, which is NOT 44% increase. Figure out that you are adding 44 to 56, but what part of 56

is 44. Well it is a fraction called 44 56ths of 56. (Like 2/3's of 3 is 2)

(or like 1/2 of 2 is 1) ( or like 44/56 of 56 is 44) ( or like 33333/888 of 888 is 33333)

It takes some getting used to.

Hope that doesn't just make it confusing.

**igloo** **myrtilles** **fourmis**

Offline