We are given that 9/10 of the state land is about 45 million acres of farm. Therefore about 1/10 of the state land is not made up of farms, therefore:
1/10 = 0.1
9/10 = 0.9
0.9 (state land) = 45 million acres
state land = 50 million acres
Hence,
not farmland = 0.1 * 50 million acres
not farmland = 5 million acres