90 because the value of the 2 multiplied gets incremented by its 2 times, for example in first difference of 2 , the value is 5. , Because 15 +5=20, and then in second difference, the value is increased to 10, double of 5, since 20 + 10 = 30, in next diff, I.e. 2x9, the value to be increased will be 20 then no. Becomes 50, now in the final diff, the value to be increased will be double of 20 I.e. 40 so the answer is 50 + 40 = 90

Hence verified