Question

Compute without a calculator:
$(72\cdot 78\cdot 85\cdot 90\cdot 98)\div (68\cdot 84\cdot 91\cdot 108).$
(There's an easier way than multiplying out the giant products $72\cdot 78\cdot 85\cdot 90\cdot 98$ and $68\cdot 84\cdot 91\cdot 108$!)

Answer 1

Take the prime factorization of each factor in the product.

$\dfrac{72 \times 78 \times 85 \times 90 \times98}{68 \times 84 \times 91 \times 108} \\\\ ~~~~~~~~ = \dfrac{(2^3\times3^2)\times(2\times3\times13)\times(5\times17)\times(2\times3^2\times5)\times(2\times7^2)}{(2^2\times17)\times(2^2\times3\times7)\times(7\times13)\times(2^2\times3^3)} \\\\ ~~~~~~~~ = \dfrac{2^6 \times 3^5 \times 5^2 \times 7^2 \times 13 \times 17}{2^6 \times 3^4 \times 7^2 \times 13 \times 17} \\\\ ~~~~~~~~ = 3 \times 5^2 = \boxed{75}$