Let a = weight of large box in pounds
Let b = weight of small box in pounds
(1) 65a + 70b=4000
(2) a+b=60
Now we can simultaneously solve these equations.
a = 60 - b. Hence 65(60 - b) + 70b = 4000 So 3900 + 5b = 4000 and 100 = 5b hence b = 20.
SImilarly a = 40. Now we can verify too by putting in the calculated values.
Total of 7 packs. 12 pencils in a pack. 7x12 = 84
Y is 72 because it is 180-108=y