I have a really smart girlfriend

**infotech** · 08-10-2007, 09:15 PM

So I'm sitting here on a Friday night trying to figure out which shaving brush I'm going to purchase next. The only problem is that I find conflicting reports of what size the C&E BBB is. I like the size of that brush and know I don't want to go smaller or much larger. I have an SR-101 and know it has an 18mm knot. So I measure the outside of the C&E brush but of course that isn't going to tell me anything because the part that's actually measured is in the handle. I sit there scratching my head when she starts paying attention to what I'm doing and puts together a simple algebra equation I completely forgot about to figure out what the knot size is of the C&E. Meanwhile I didn't let on what I thought it was.

SR-101 = 2.25 inches around the base of the knot as far down as I can measure around the hair

18mm knot

C&E BBB = 2.75 inches around the base of the knot as far down as I can measure around the hair

x mm knot

2.25 in = 18mm
2.75 in = x mm

convert inches to mm

57.15 mm = 18mm
69.85 mm = x mm

Cross multiply move the x over divide by 57.15

57.15x/57.15 = 1257.3/57.15

x = 22 mm

9th grade algebra at its greatest. And all this time I thought algebra was a waste of time.

Based upon these findings I believe I'll go with something between 22-24 mm; more than likely an SR 3124 or 22. Is there much of a difference in 2 mm?

**echophonic** · 08-10-2007, 10:52 PM

Yup. That's like the one final remaining algebra equation still left in my head, and it's a darn handy one at that.

Horray for women being smarter than men!

(however, there are other things about being a guy that kinda rock...)

**bjrn** · 08-11-2007, 02:59 AM

Hooray for maths! Although you could have just done circumference/pi :P

**galpman** · 08-11-2007, 04:48 AM

Class is now in session!!

**drumana** · 08-11-2007, 05:40 AM

Dang.

Forgot that one. . . probably never learned it actually.

**ouch** · 08-11-2007, 05:54 AM

If you had the circumference, why didn't you just divide by pi?

**infotech** · 08-11-2007, 07:07 AM

Originally Posted by ouch

If you had the circumference, why didn't you just divide by pi?

We didn't have any pie

**Austin** · 08-11-2007, 07:10 AM

I like cherry pie!

**ouch** · 08-11-2007, 07:12 AM

Now ya done wnet and woke up Nick.

Hopefully, he won't make it past the "smart girlfriend" title.

**bigDee** · 08-14-2007, 02:23 AM

You didn't need to do any converting before solving for x.

**analog_kid** · 08-14-2007, 08:55 AM

I too was promised there would be no math!

**rickw** · 08-14-2007, 09:46 AM

The correct answer is 19mm.

2.75/pi *22.4 =19.6

**bjrn** · 08-14-2007, 10:03 AM

Nope, one inch is 25.4 mm, not 22.4

**ouch** · 08-14-2007, 10:05 AM

Originally Posted by rickw

The correct answer is 19mm.

2.75/pi *22.4 =19.6

22.4??? Er.......

Also, a good approximation for pi is 355/113. Plug it in to your calculator and see.

**rickw** · 08-14-2007, 12:22 PM

22.4 is mm/inch. pi is 3.1416

**greenerock** · 08-14-2007, 12:32 PM

But is she smarter than a 5th grader????

**FloppyShoes** · 08-14-2007, 01:00 PM

approximately 25.4 mm/in, this is accurate enough because your initial measurements had 3 significant figures.

ouch is right about his approximation of pi, that's a good and easy approximation, it gives you 7 significant figures. Numerically there are better methods, but this is by far the shortest. I can't think of any instances when more then 7 sig figs is needed.

**rickw** · 08-14-2007, 01:28 PM

Originally Posted by FloppyShoes

approximately 25.4 mm/in, this is accurate enough because your initial measurements had 3 significant figures.

You're correct. I don't know where I came up with that.

**FloppyShoes** · 08-14-2007, 01:39 PM

Originally Posted by rickw

You're correct. I don't know where I came up with that.

I had better be

I'm a soon-to-be building engineer. If you're off in your calcs on shaving brush knots, no biggie. If you're off on load calculations in a large steel structure... I won't even go there.