Simple, comparing the two formulas, the next one is only three triangles more than the last one, and the sum is 60-36=24.

This 24 is the sum of three triangles, so if 24 is divided by 3=8, the triangle is 8.

Put triangle =8 in the top formula, the sum of two triangles is equal to 16, two squares are equal to 36- 16=20, and then the average is 20 divided by 2= 10.

So: triangle =8 square = 10.