What is the smallest positive integer that is the sum of a multiple of $15$ and a multiple of $21$? (Remember that multiples can be negative.)
(15)(3) + (21)(–2)
45 – 42 = 3
You can get it another way.
(21)(3) + (15)(–4)
63 – 60 = 3
.
(21)(1/7) + (15)(–1/5)
3 – 3 = 0
Nowhere did it say that the multiple had to be an integer.
The only problem I see with this is the question, is zero positive?
+27 
