5a - 10 = 4a - 4; 5a - 4a = 10-4; a =6
Answer:
Step-by-step explanation:
To find the number of half lives divide the total number of days by the number of days for 1 half live:
1104 / 138 = 8
There are 8 half lives.
To find the total amount left multiply the starting amount by 1/2 to the 8th power:
38 * 0.5^8 = 0.148 = 0.15 grams
The answer is the last one.
Total cost of lessons : 305.50
Deduct the first two lessons which costs 12.75 each from the total cost.
12.75 * 2 = 25.50 is the total cost of the first two lessons.
305.50 - 25.50 = 280 total cost from the 3rd lesson upwards.
Since, it was stated that after the first two lessons, all other lessons cost 20 each lesson. We divide the remaining 280 by 20 to get the number of lessons taken.
280 / 20 = 14 lessons.
To get the total number of lessons Leah has; we add the first two lessons to the quotient, which is 14 lessons.
2 lessons + 14 lessons = 16 lessons in total.
To check: Multiply the corresponding lessons to its cost and check if the total cost arrived is equal to the total cost stated in the problem.
2 lessons * 12.75 = 25.50
14 lessons * 20.00 = 280.00
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16 lessons 305.50
