156 weather

First of all, the molecular law is an odd number starting from 7: 7,9, 1 1, 13, 15.

Secondly, the symbol is: mixed addition and subtraction.

Finally look at the denominator. The denominator is actually the sum of the previous denominator+the current numerator minus one. take for example

The second denominator is 20= 12 (the first denominator) +9 (the second molecule is the current molecule)-1.

The third denominator is 30=20 (second denominator)+1 1 (the third numerator is the current numerator)-1.

. . . . . . etc

42=30+ 13- 1

56=42+ 15- 1