Probability distributions -Bulb failure

the length of time that light bulbs will be functional is normally distributed with a mean of 800 hours and a standard deviation of 160 hours. If testing determines that 0.3% of the bulbs fail within the first x hours then x in hours is:

The corresponding z -score for 0.3% or 0.003
0.5 -0.003= 0.497
z-score will be -2.75
(x-800)/ 160 = -2.75

x= 360 hrs.

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The art of problem solving - answer

At a certain time, Janice notices that her digital watch read a

minutes after two 0' clock. 15 min. later, it reads b min. after

three o' clock. She is amused to note that a is six times greater

than b. What time was it when she looked at her watch for the

second time?

2:54 and 3:09
a+15 = 60+b
as time has changed from 2 to 3 ( 1 hr = 60 minutes)
and we have a = 6b

so use the above two equations we get
5b=45 and b =9
therefore b =09 and a = 9*6 = 54
the times are 2:54 and 3:09

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Normal Distribution problem

The length of time that light bulbs will be functional is normally distributed with a mean of 800 hours and a standard deviation of 160 hours. If testing determines that 0.3% of the bulbs fail within the first x hours then x in hours is:

The corresponding z -score for 0.3% or 0.003
0.5 -0.003= 0.497
z-score will be -2.75
(x-800)/ 160 = -2.75

x= 360 hrs.

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Numbers -Word Problems

The number 210 can be written as the sum of consecutive positive integers in several ways. When written as the sum of the greatest possible number of consecutive positive integers, what is the largest of these integers?

Sum to n terms 1,2,3,.........n is n(n+1) / 2.This will give us greated number of consecutive positive integers.

n(n+1)/2 = 210
so n(n+1)= 420
20*21= 420
so answer is 20

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Math Word Problems-Car Uphill downhill

A car travels downhill at 72 mph (miles per hour), on the level at 63 mph, and uphill at
only 56 mph. The car takes 4 hours to travel from town A to town B. The return trip
takes 4 hours and 40 minutes. Find the distance (in miles) between the two towns

Let the total distance travelled downhill, on the level, and uphill, on the outbound journey, be x, y, and z, respectively.
The time taken to travel a distance s at speed v is s/v.

Hence, for the outbound journey

x/72 + y/63 + z/56 = 4

While for the return journey, which we assume to be along the same roads

x/56 + y/63 + z/72 = 14/3

It may at first seem that we have too little information to solve the puzzle. After all, two equations in three unknowns do not have a unique solution. However, we are not asked for the values of x, y, and z, individually; but for the value of x + y + z.

Multiplying both equations by the least common multiple of denominators 56, 63, and 72, we obtain

7x + 8y + 9z = 4 · 7 · 8 · 9
9x + 8y + 7z = (14/3) · 7 · 8 · 9

Now it is clear that we should add the equations, yielding

16(x + y + z) = (26/3) · 7 · 8 · 9

Therefore x + y + z = 273; the distance between the two towns is 273 miles.

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Euclid's Algorithm

A pair of positive integers (x, y) satisfies the equation 31x + 29 y = 1125. What is x + y? ans - x+y=37

31x+29y=1125

31x+29y=1 we get for x=-14 and y=15

general solution is x =-14*1125+29*t y = 15*1125-31t

14*1125/29 <= t<= 15*1125/31 t is an integer
we get t as 544

plug in t back in the equations above
to get x =26 and y=11
so x+y=37

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