2 answers
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let the time taken be t seconds
2 + .3t = 5 - .12t
.42t = 3
t = 3/.42 = 7.14 seconds
check:
after 7.14 seconds, volume of smaller balloon = 2 + 7.14(.3) = 4.14
after 7.14 seconds, volume larger ballon = 5 - 7.14(.12) = 4.14
OK then!