You would do:
1) Find the amount that it has increased by subtracting old from new number
26-18= 8
2) Take your old number/ original number (18) and divide it into 8.
8/18= 0.44... (repeats 4 FOREVER)
3) Multiply 0.44 by 100 to get a percent.
0.44*100= 44 (round to the nearest whole)
4) Final answer= 44%