Question 588227
The simple interest (Is)in a savings account varies jointly as the Principal (P) and the time (T). After 3 months, 8000 pesos deposit earned 100 pesos in interest. How much will a 10,000 pesos principal earn in 5 months?
This is about Variation. But this kind of variation is a joint variation. Its formula to solve it is y=kxz. 
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You are correct. 
k = 1/240
AND
Is for 10,000 pesos in 5 months SHOULD be bigger.
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How much will a 10,000 pesos principal earn in 5 months?
Is = (1/240)PT
Is = (1/240)(10000)(5)
Is = (1/24)(1000)(5)
Is = (1/24)(5000)
Is = 5000/24
Is = 208.33 pesos