The equation is y = 20(1.40)^x
where x is the number of days and y is the future amount after x days go by
The 1.40 is from the fact that 1+r = 1+0.40 = 1.40, where r is the decimal form of 40%, so r = 0.40
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Plug in x = 7 to get
y = 20(1.40)^x
y = 20(1.40)^7
y = 210.827008
y = 211
We round to the nearest whole number since it doesn't make much sense to have a fractional portion of an algae. After 7 days, there are about 211 algae.