https://pokemondb.net/pokebase/287135/when-a-garchomp-mega-evolves-how-much-speed-does-it-lose?show=345260#c345260
Generally, there's no need to comment on old questions and point out that they're against the rules. Also, I personally understand the question as asking for how much the reduction is in a percent so it can be applied to any number Garchomp's speed stat might be, rather than just the base speed, which is not on Garchomp's page.