This problem looks familiar. It may have been posted before. However, the solution is relatively easy, if we use a bit of logic. It goes like this:
$1 - $0.75 =$0.25 Difference in price per bushel makes a difference of:
$80 - $70 =$10 an acre in rent. Therefore, the rent paid in wheat is: $10 / $0.25 =40 bushels per acre.
40 bushels x $1 =$40. So that the rent per acre paid in cash would be:
$80 - $40 =$40 per acre. Hence, the number of acres is:
$800 / $40 =20 acres - the size of Bob's farm.