What about it?

Number Theory Level 2

If $A=123456789$ and $B=987654321$ , what are the last two digits of $A\times B$ ?

2 solutions

Finn C
May 18, 2016

If you want to simplify Sam Bealing's solution further, instead of multiplying 2 digit numbers you can do:

9 x 1 = 9 (our last digit)

and

2 x 8 = 16, although we only take the 6

Therefore, our answer is 69

Nice, didn't think of that. Though you may want to point out that it only works since 9x1 is less than 10

Alex Li - 4 years, 9 months ago

Sam Bealing
May 10, 2016

We need only multiply the last two digits of each number to find the last two digits of $A \times B$ :

$21 \times 89=1869$

The last two digits are therfore:

$\boxed{69}$

Nice solution

Fidel Simanjuntak - 5 years, 1 month ago