As you can see, he uses one square for Stage 1, three squares for Stage 2 and six for Stage 3. How many squares should he use for the sixth stage?

21 squares

Step-by-step explanation:

1 = 1

2 = 3

3 = 6

The pattern follows squares use in the previous stage + new stage = squares use the new stage

Squares used in stage 2 = Squares used in stage 1 + stage 2

= 1 + 2

= 3 squares

Squares used in stage 3 = Squares used in stage 2 + stage 3

= 3 + 3

= 6 squares

Squares used in stage 4 = Squares used in stage 3 + stage 4

= 6 + 4

= 10 squares

Squares used in stage 5 = Squares used in stage 4 + stage 5

= 10 + 5

= 15 squares

Squares used in stage 6 = Squares used in stage 5 + stage 6

= 15 + 6

= 21 squares

4 = 10

5 = 15

6 = 21

the multiples of 15 are 15, 30,45, 60, 75, 90, 105, 120, 135, 150, 165, 180, 195,   210, 225, 240, 255, 270, 285, 300, 315, 330, 345, 360, 375,   390, 405, 420, 435, 450, 465, 480, 495, 510, 525, 540, 555, 570, 585, 600, 615, 630, 645, 660, 675, 690, 705, 720, and 735.

the factors of 15 are 1, 3, 5, and 15.

hope that , fam!

-une, your fren : )