1st row: Cost = $420
2nd row: Cost = $420(0.9)
3rd row: Cost = $420(0.9)²
By induction, the cost in the n-th row is
Cost = $420(0.9)ⁿ⁻¹
When the cost is $75, the row number, n, is given by
420(0.9)ⁿ⁻¹ = 75
(0.9)ⁿ⁻¹ = 75/420 = 0.1786
Take logs of each side.

Therefore
n = 17 (nearest integer)
Answer: Row 17