Then the average of the four heights will be: (70+71+72+x)/4 Simplify it to (213+x)/4
This average has to be at least 73 inches. Meaning, it has to be greater than or equal to 73. (I don't have the sign for greater/equal so I'll type >=)
(213+x)/4 >= 73 This is the inequality. Just solve for x, and you have the answer. (213+x) >= 292 x >= 79
Another way: (I think it's easier to do it like this in my head) You need average 73. Look at the heights of the plants you have now: how far are they from 73? The first is 3 less, the second is 2 less, and the third is 1 less. The 'total deficit'' is 6. So to make the average be 73, you need to cancel out the deficit by making an equal overage (by 6). If you can only add one more plant... that plant has to be 6 inches higher than the average you want. So 73+6=79.