Answer:
a = 50 houses
b = 45 houses
Step-by-step explanation:
Given
Number of houses called Model A = a
Number of houses called Model B = b
Total of single windows = 1800
Total of double windows = 375
then we have the system of equations
18a + 20b = 1800 (I)
3a + 5b = 375 (II)
Solving the system by whatever method we prefer, we obtain
(I) a = (1800 - 20b)/18
then (II)
3((1800 - 20b)/18) + 5b = 375
⇒ 300 - (10/3)*b + 5b = 375
⇒ (5/3)*b = 75
⇒ b = 45 houses
then
a = (1800 - 20*45)/18
⇒ a = 50 houses