Question 1207614
<pre>
Hi
A group of girls had an average height of 155cm.3 more girls joined the group and the average height became 157cm. If the 3 girls had the same height of 163cm each, how many girls were there at first.

Let original number of girls be G
Then height of original number of girls = 155G
After another 3 joined the group, height of the entire group was, 155G + 3(163) = 155G + 489
Number in group after 3 more joined: G + 3
We then get: {{{matrix(2,3, matrix(1,7, Height, of, group, after, 3, more, joined)/matrix(1,7, Number, in, group, after, 3, more, joined), "=", matrix(1,6, Average, height, after, 3, more, joined), (155G + 489)/(G + 3), "=", 157)}}}
                                           155G + 489 = 157(G + 3) ----- Cross-multiplying
                                           155G + 489 = 157G + 471
                                          155G - 157G = 471 - 489
                                                 - 2G = - 18
                <font color = red><font size = 4><b>Original number of girls</font></font></b>, or {{{highlight_green(matrix(1,5, G, "=", (- 18)/(- 2), "=", highlight(9)))}}}</pre>