Respuesta :

The answer would most defiantly be “21”, choice c “21”
KurdishPotato

Answer:

21

Step-by-step explanation:

The formula for triangular numbers is n×(n+1)÷2 (n is the step of next number)

we have 1, 3, 6, 10, 15 so the next step would be 6th

6×(6+1)÷2 = 21

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