Answer:
110 marbles
side 14
Step-by-step explanation:
Draw triangles using marbles as the sides.
If the side is 2 marbles long, the triangle uses 3 marbles. 1 + 2 = 3
If the side is 3 marbles long, the triangle uses 6 marbles. 1 + 2 + 3 = 6
If the side is 4 marbles long, the triangle uses 10 marbles. 1 + 2 + 3 + 4 = 10
If the side is n marbles long, the triangle uses n(n + 1)/2 marbles.
These numbers are called triangular numbers.
We are looking for the side, n.
n(n + 1)/2 + 5 = (n + 1)(n + 2)/2 - 10
n^2 + n + 10 = n^2 + 3n + 2 - 20
n + 10 = 3n - 18
-n = -14
n = 14
For n = 14
n(n + 1)/2 =
= 14(15)/2
= 105
For n = 15
n(n + 1)/2 =
= 15(16)/2
= 120
105 + 5 = 120 - 10
You have 110 marbles. If you had 5 fewer, 105, you'd have a triangle with sides 14. If you had 10 more, 120, you'd have a triangle with sides 15. Since you have 110, you have 5 too many for side 14 and 10 too few for side 15.
The sides of the triangle is 14.
