The triangular numbers are the sums of the natural numbers.
T₁ = 1
T₂ = 1 + 2 = 3
T₃ = 1 + 2 + 3 = 6
T₄ = T₃ + 4 = 10
T₅ = T₄ + 5 = 15
T₆ = T₅ + 6 = 21
T₇ = 28
T₈ = 36
T₉ = 45
T₁₀ = 55
Let's subtract each from 49 and see if the result is on the list:
49-1=48, nope
49-3=46, 49-6=43, then 39, 34, 49-21=28, winner
The rest won't give any new possibilities.
Answer: 21 and 28
