Answer:
6
Step-by-step explanation:
Let the first, second and third numbers be a, b and c respectively.
a +b +c= 72 -----(1)
b= 7a -----(2)
c= b -18 -----(3)
Substitute (2) into (3):
c= 7a -18 -----(4)
Substitute (2) and (4) into (1):
a +7a +7a -18= 72
Simplify:
15a -18= 72
Add 18 to both sides of the equation:
15a= 72 +18
15a= 90
Divide both sides by 15:
a= 90 ÷15
a= 6
Thus, the first number is 6.
