Respuesta :
Let's call [tex] x [/tex] the desired amount. We know that the 80% of x is 336. Since we compute the 80% of something by multiplying it by 0.8, we have
[tex] 0.8x = 336 \implies x = \frac{336}{0.8} = 420 [/tex].
So, they still need to collect
[tex] 420-336 = 84 [/tex] dollars.
At a rate of 21 dollar per car, they still have to wash
[tex] \frac{84}{21} = 4 [/tex] cars.
What hey want to collect is X
80% of X = $ 336
Amount raised by friends 1s $336.00
1% of X = $ 336/80 = $ 42
100% 0f X = $ 42 x 100 = $ 420
Amount needed = $ 420 - 336 = $ 84
Earnings for each car washed = $ 21
Number of cars needed to wash is 84/21 = 4