for above query before answering go through the relevant rules of pension as above said is 20 years and it is wrong as 10 years is sufficient to get pension and i am giving you the paragraph for your ready reference
The minimum eligibility period for receipt of pension is 10 years. A Central Government servant retiring in accordance with the Pension Rules is entitled to receive superannuation pension on completion of at least 10 years of qualifying service.
In the case of Family Pension the widow is eligible to receive pension on death of her spouse after completion of one year of continuous service or before even completion of one year if the Government servant had been examined by the appropriate Medical Authority and declared fit for Government service.
W.e.f 1.1.2006, Pension is calculated with reference to average emoluments namely, the average of the basic pay drawn during the last 10 months of the service or last basic pay drawn whichever is beneficial. Full pension with 10/20 years of qualifying service is 50% of the average emoluments or last basic pay drawn whichever is beneficial. Before 1.1.2006, for qualifying service of less than 33 years, amount of pension was proportionate to the actual qualifying service broken into completed half-year periods. For example, if total qualifying service is 30 years and 4 months (i.e. 61 half-year periods), pension will be calculated as under:-
Pension amount = R/2(X)61/66
where R represents average reckonable emoluments for last 10 months of qualifying service or the last pay drawn as opted by the govt servant.
Minimum pension presently is Rs. 3500 per month. Maximum limit on pension is 50% of the highest pay in the Government of India (presently Rs. 45,000) per month. Pension is payable up to and including the date of death.
source : https://pensionersportal.gov.in/retire-benefit.asp