She saves 1/10 of her salary. That's the simplified version, it can also be written 10/100. Fractions, decimals, and percentages are all interchangeable.
The given formula is f(t) = 0.4t
They want to solve for f(10), replace t in the formula with 10"
f(10) = 0.4(10)
Multiply:
f(10) = 4 centimeters.
