Hello from MrBillDoesMath! 
 
Answer: 
180000
 
Discussion: 
 Let H = house and L = lot. The Question tells us that H + L = 420000 and L = 3/4H. We are asked to solve for L
H + L = 420000 => L = 3/4H implies that H =4/3L 
(4/3)L + L = 420000 => (4/3) + 1 = 7/3
(7/3)L = 420000 => multiply both sides by 3/7
(3/7)(7/3)L = (3/7)420000 => as (3/7)(7/3) = 1
L = 180000
Check: 
H + L = 420000
H + 180000 = 420000
H = 420000 - 180000
H = =240000
Is L = 3/4H ?
Does 180000 = 3/4 ( 240000) ?
Does 180000 = (3* 240000)/4 = 720000/4 = 180000 Yes! so the answers check!
 
Thank you, 
 
MrB