Flopjie changes £50 into Mexican pesos. The exchange rate is £1 = 25 Mexican pesos. How many pesos does she receive?

Step 1: The function is given as

$$f(x)=\int_{a}^{x} f(t)\,dt.$$

Step 2: By the Fundamental Theorem of Calculus (FTC),
if $F(x)=\int_{a}^{x} g(t)\,dt,$
then $F'(x)=g(x).$

Step 3: Here, $g(t)=f(t),$
so substitute to get

**$\boxed{f'(x)=f(x)}$**