Question

TRIANGULAR NUMBERS By arranging
bowling pins in their proper order, a
triangle is created. The table below
shows the number of rows of bowling
pins and the total number of pins in the triangle. The formula:

n * (n+1) / 2

can be used to find t, the total number of items in n rows. How many bowling pins would there be if 10 rows of pins were set up?

Rows 1 2 3 4
Pins 1 3 6 10

Answer 1

check your formula for row 4 , (n=4)
number of pins = 4(5)/2 = 10
checks out with your chart

so if n=10
number of pins = 10(11)/2 = 55

Answer 2

Thanks Reiny!