Let x be the number of gallons of a 10 percent ammonia solution that should be mixed. Then, the amount of ammonia in x gallons of a 10 percent solution is 0.1x gallons. The amount of ammonia in 50 gallons of a 30 percent solution is 0.3 * 50 = 15 gallons. The total amount of ammonia in the final 15 percent solution should be 0.15 * 15 = 2.25 gallons. Therefore, we can set up the following equation:

```

0.1x + 15 = 2.25

```

Solving for x, we get:

```

x = (2.25 - 15) / 0.1 = 7.5

```

Therefore, 7.5 gallons of a 10 percent ammonia solution should be mixed with 50 gallons of a 30 percent solution to make 15 solution.