1365.11 – Decimals On Screen


How many different numbers between 18\dfrac{1}{8} and 12\dfrac{1}{2}, inclusive, can be displayed on your shiny new 8-place digital calculator app?


Solution

We start with

1/8:.12500000.1/8: .1 2 5 0 0 0 0 0.

Think of this as: .125 _ _ _ _ _.1 2 5\ \_\ \_\ \_\ \_\ \_ , where there are 10 possible digits for each blank, thus 10510^5 possible 8-place decimal numbers greater than 18\dfrac{1}{8} and beginning with .125.125. There is a similar result for .126 _ _ _ _ _.126\ \_\ \_\ \_\ \_\ \_, and for .127 _ _ _ _ _.127\ \_\ \_\ \_\ \_\ \_, and so on up to .129 _ _ _ _ _.129\ \_\ \_\ \_\ \_\ \_. This makes 51055 \cdot 10^5 numbers so far.

Next, for .13 _ _ _ _ _ _.13\ \_\ \_\ \_\ \_\ \_\ \_, .14 _ _ _ _ _ _.14\ \_\ \_\ \_\ \_\ \_\ \_, and so on up to .19 _ _ _ _ _ _.19\ \_\ \_\ \_\ \_\ \_\ \_, we get 71067 \cdot 10^6 more. And for .2 _ _ _ _ _ _ _.2\ \_\ \_\ \_\ \_\ \_\ \_\ \_, .3 _ _ _ _ _ _ _.3\ \_\ \_\ \_\ \_\ \_\ \_\ \_, and .4 _ _ _ _ _ _ _.4\ \_\ \_\ \_\ \_\ \_\ \_\ \_, we get 31073 \cdot 10^7 more.

Finally, there is .5 itself, i.e., 12\dfrac{1}{2}. One possibility.

Now add:

(5105)+(7106)+(3107)+1=37,500,001.(5 \cdot 10^5) + (7 \cdot 10^6) + (3 \cdot 10^7) + 1 = 37,500,001.

That's a lot!