math
posted by Debbie on .
How many threedigit integers are multiples of both 4 and 6 and have a unit digit of 2?

to be multiples of 4 and 6, they must be multiples of 12, the LCM
The smallest 3digit multiple of 12 is
108 , then 120 , 132 .. .996
consider this to be an arithmetic sequence
a = 108, d = 12 , n = ?  the number of terms
t(n) = a + (n1)d
996 = 108 + 12(n1)
888 = 12n  24
12n = 912
n = 76
There are 76 of them 
Since 12n = 12(5m+1)= 60m +12, our goal is to now count integers m such that 100 less than or equal to 60m +12<1000. Subtracing 12 from each part of the inequality, we get 88 lees than or equal to 60m<988. Dividing by 60, we get 2 less than or equal to m less than or equal to 16, so there are 161=15 values for the integer m that give us the threedigit multiples of 4 and 6 that have units digits of 2.