Hi guys! I’m a bit confused about divisibility rules. In part of the video, Greg takes the prime factorization of 1980. He says that 1980 is divisible by 9 and 10, so it should be divisible by 90, and proceeds to divide by 90. I chose the numbers 4 and 10, so it should be divisible by 40 (ends in 0 and 80/4 = 20), but when I put it into my calculator I get 49.5 which isn’t an integer. Does this mean if two numbers are divisible by 1980 (or any integer), we can’t multiply those two factors together to get another divisible number.
It worked for 90 and not for 40 because 9 (3x3) and 10 (2x5) don’t share any factors. But 4 (2x2) and 10 (2x5) do.
yes, because they might share factors
To give a simpler example,
Take 80, 8 divides 80, and 4 divides 80. But 32 (8*4) doesn’t because they share factors.