think it this way. 120/3=40. you need three consecutive numbers, which means those numbers are n1+n2+n3=120 and middle number n2 is gotten by dividing the number 120 with how many numbers you sum, in this case 3. the smaller number n1 is n2-1 and bigger n3 is n2+1
n1=40-1
n2=40+0
n3=40+1
