What does the triangle in front of a variable mean and how is it used?

Ammonia molecules have three hydrogen atoms and one nitrogen atom.how many of each atom arein five molecules of ammonia

The measure of an angle is 117°. what is the measure of a supplementary angle?

In a two-digit number the units' digit is 7 more than the tens' digit. the number with digits reversed is three times as large as the sum of the original number and the two digits. find the number.