Thomas from Beechwood Park School didn't stop at the blocks with J on them:

C=6
D=10
E=15
F=21
G=28
H=36
I=45
J=55
K=66
L=78
M=91
N=105
O=120
P=136
Q=153
R=171
S=190
T=210
U=231
V=253
W=276
X=300
Y=325
Z=351

Chloe and Georgia from Moorgate CP School, and Matt from St Andrew's School also calculated carefully.

Class 5 from Spring Grove Primary School in Hounslow told us:

These numbers are called triangle numbers because you would be able to arrange the number of blocks into a triangle formation. It would also work if they were counters as well.

That's right! Thank you.