How much water should be mixed with 237 ml of ammonia, whose strength is 100%, in order to create a mixture that is diluted to a 75% strength?

In this case, you know your solution volume( 237ml) and your solution strength (100%). You want to make a mixture that has 75% strength, which means it has 75% of ammonia and 25% is the diluting water. Then the calculation would be:

Water= 25%/75% x 237ml = 79ml of water. 

If a hydrogen atom is neutral, it must also have  one electron. an oxygen atom (o) has an atomic number of eight (8). this means a neutral oxygen atom has eight protons and eight electrons. the element actium (ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom